In most cases, arbitrary order (together with "gingerness" induced by inter-component references) is sufficient for instantiating the order of sibling components. However, in some cases, for example, where the only result of one instantiation is a set of (potentially unknown) context tags whose purpose is to guide the instantiation of the other, this natural order is not sufficient and one component must definitely precede its sibling, even though there is not the possibility of writing a dependence (fictitious or otherwise) that might force this ordering.
In these (sparing) cases it would be useful to attach a "priority" annotation to a subcomponent record which will provide a hint to the system as to relative ordering. These "priority" fields will have the identical values and semantics as "priority" records attached to event listeners (which are similarly sparingly applied, but crucial when they are).