That was clear from the original terms of s.8 of the 1991 Act, and, even in its present day amended form, the 1991 Act ousts the jurisdiction of the courts in most cases:
"(3) Except as provided in subsection (3A), in any case where subsection (1) applies, no court shall exercise any power which it would otherwise have to make, vary or revive any maintenance order in relation to the child and non-resident parent concerned."
There are exceptions, such as educational costs, and the costs associated with a child’s disability but these are very much the exceptions to the general rule. The exception which has not been widely used in Scotland is the one found in s.8(6) of the 1991 Act whereby the court can retain jurisdiction if the paying parent earns, or is deemed to earn, more than £3,000 gross per week, so long as a maintenance calculation is in effect:
"(6) This section shall not prevent a court from exercising any power which it has to make a maintenance order in relation to a child if–
(a) a maintenance calculation is in force with respect to the child;
(b) the non-resident parent’s gross weekly income exceeds the figure referred to in paragraph 10(3) of Schedule 1 (as it has effect from time to time pursuant to regulations made under paragraph 10A(1)(b)); and
(c) the court is satisfied that the circumstances of the case make it appropriate for the non-resident parent to make or secure the making of periodical payments under a maintenance order in addition to the child support maintenance payable by him in accordance with the maintenance calculation."
The exception has been much more widely used in England, with its much bigger population of wealthier paying parents. The other factor which makes the exception more common in England is the consent order — broadly equivalent to our more flexible registered minute of agreement — because in cases in which Scottish parties would enter into an agreement without reference to the court, English parties apply to the court for an order in agreed terms, applying the exception in s.8(5) of the 1991 Act:
"(5) The Lord Chancellor or in relation to Scotland the Lord Advocate may by order provide that, in such circumstances as may be specified by the order, this section shall not prevent a court from exercising any power which it has to make a maintenance order in relation to a child if–
(a) a written agreement (whether or not enforceable) provides for the making, or securing, by an absent parent non-resident parent of the child of periodical payments to or for the benefit of the child; and
(b) the maintenance order which the court makes is, in all material respects, in the same terms as that agreement."
It’s not unknown for a Scottish minute of agreement to provide for a general amount of money to be paid to an ex-spouse, without apportionment between child aliment and periodical allowance, though it is usually unwise to make such a provision in the light of s.4(10)(aa) of the 1991 Act allowing a relevant clause to be overturned after a year:
"(10) No application may be made at any time under this section with respect to a qualifying child or any qualifying children if–
(aa) a maintenance order made on or after the date prescribed for the purposes of paragraph (a) is in force in respect of them, but has been so for less than the period of one year beginning with the date on which it was made;
(ab) a maintenance agreement–
(i) made on or after the date prescribed for the purposes of paragraph (a); and*S.L.T. 62
(ii) registered for execution in the Books of Council and Session or the sheriff court books, is in force in respect of them, but has been so for less than the period of one year beginning with the date on which it was made."
An agreement in those terms, though, is certainly possible and enforceable so long as no maintenance calculation has (yet) been made in respect of the children. The question is whether a general clause in a Scottish minute of agreement can be framed in such terms so as to withstand a maintenance calculation being undertaken.
The differences and peculiarities of the English system can help to illustrate a point which could arise in a Scottish case if an agreement were framed along English lines.
The English court in D v R  EWHC 4306 (Fam) looked at a freestanding claim for child maintenance in terms of section 8(6) of the 1991 Act. There was a maintenance calculation in force which, according to the mother greatly understated the actual earnings of the non-resident father, who was employed by a UK company in Jersey. The court held:
"It is my very clear and firm view, and so I hold, that the top up jurisdiction under section 8(6) of the Child Support Act 1991 is not available unless the Child Maintenance Service have themselves assessed the gross weekly income as being or exceeding £3,000 per week or (which comes to same thing) have made a maximum maintenance calculation currently in the sum of £294 a week or £15,288 per annum."
The court held that the only avenue available to the mother was to make an application to the First-tier Tribunal for the maintenance calculation to be reviewed. The view has been expressed that this decision was wrong and that it would be open to any court — certainly any Scottish court — to look more broadly at the income and resources of the non-resident parent in assessing liability for aliment under s.4 of the Family Law (Scotland) Act 1985 (see J M Fotheringham, Aliment, 2016 S.L.T. (News) at p.163).
A different aspect of the issue has recently been considered by the High Court in England in the case of AB v CD  EWHC 3164 (Fam), on appeal from the Central Family Court. In that case, the parties were both solicitors and had represented themselves at first instance. There were substantial funds allowing for a generous maintenance award. The father, who was the non-resident parent, was earning a salary of £320,000 pa and, according to the court, there was a "real possibility" that his earnings would rise considerably in the future. He was very clearly going to be vulnerable to a top-up claim.
At the time of the court’s order, no application for child support maintenance had actually been made and thus, the gateway jurisdiction under s.8(6) of the 1991 Act was not in play. An application had been made by the time the case reached the High Court, but decision under appeal had been made without it. The court at first instance had addressed the matter in terms of a so-called ‘Segal order’. Such an order allows for a global maintenance order to be made for a spouse and children together, without apportionment, even in the absence of any maintenance calculation, whether or not at the maximum capped level by the statutory authority. An important element of a Segal order is that when a maintenance calculation is made, the amount due under the global maintenance order is reduced pound for pound by the amount due under the maintenance calculation so that, in effect, the amount of maintenance due overall is the same.
The husband challenged the order claiming that it was incompetent to order maintenance (including child maintenance) in the absence of a maintenance calculation, but the High Court rejected his argument. Referring to how (English) courts have historically operated, the High Court took the view that a Segal order is legitimate even though it can be described as "just within the bounds" of that legitimacy, albeit that such orders must have a "substantial ingredient of spousal support", for they do not "challenge or seek to oust the jurisdiction" of the Child Maintenance Service.
This appears to have overstepped what District Judge Segal envisaged by a Segal order in so far as he felt it could only be justified as a very short term measure pending application for and the calculation of a maintenance liability. The High Court stressed that no free standing order for maintenance of the children could be possible without a maintenance calculation, but that it was nevertheless appropriate that a global maintenance*S.L.T. 63 order should be made on the condition that the husband’s income crossed the threshold for a maximum assessment and the amount of any later child support maintenance calculation would expressly be deducted from the liability under the court’s global maintenance order.
To some extent AB v CD emphasised the correctness of D v R and also TW (Children), Re  EWHC 3054 (Fam), although the latter case dealt with the variation of an earlier consent order rather than the imposition of an originating order. Variation of an existing order is competent by virtue of s.8(3A) of the 1991 Act provided that the conditions of subs (a) and (b) are satisfied. The date prescribed in respect of the subsections is 3 March 2003 so there are unlikely to be many existing cases which are excluded. The important point is that absent consent of the parties, and absent any maintenance calculation in terms of s.8(6)(a) and (b) the court has no jurisdiction to make any new order for aliment in any part of any case which falls within the scope of s.44 of the 1991 Act. There is no restriction on an action for aliment for a child if the alimentary liability is derived from s.(1)(d) of the Family Law (Scotland) Act 1985 (see also Dorney-Kingdom v Dorney-Kingdom  2 F.L.R. 855).
So far as the authors are aware, there have been no cases involving global aliment or periodical allowance incorporating specific sums for the maintenance of children in Scotland. There is the provision in s.4(4) of the Family Law (Scotland) Act 1985 that an award for aliment for a child may include such provision as it considers to be in all the circumstances reasonable in respect of the expenses incurred wholly or partly by the person having care of the child for the purpose of caring for the child (authors’ emphasis). That is not at all the same thing as a global maintenance order along Segal lines, and in any event, no such order under s.4 of the 1985 Act could exist in the absence of the gateway jurisdiction of s.8(6) of the 1991 Act.
It is submitted that the English policy of allowing a global maintenance order in Segal terms should not be followed in Scotland, even to the extent of incorporating its lack of apportionment in a registered minute of agreement. The terms of s.8 exclude such a policy. The justification given by the High Court in AB v CD seems to depend on convenience and utility rather than law, and upon the fact that it has generally been allowed over many years without challenge.
One can imagine a case in which the parties agree to the easier enforceability (especially where there is a foreign element) of a court order as opposed to that of a registered minute. The court could in these circumstances make a competent order without reference to the amount of a notional calculation of child support maintenance. However, the element of that order dealing with any aliment which could have been covered by statutory child support maintenance will be vulnerable to s.4(10)(aa) of the 1991 Act and will be rare in practice.