The docs have this to say on the matter:
The IfcMaterialLayerSetUsage is always assigned to an individual occurrence object (and only to relevant subtypes of IfcElement). The IfcMaterialLayerSet, referenced by ForLayerSet , can however be shared among several occurrence objects. If the element type is available (in other words, an instance of the relevant subtype of IfcElementType exists), then the IfcMaterialLayerSet can be assigned to the element type. The assignment between a subtype of IfcElement and the IfcMaterialLayerSetUsage is handled by IfcRelAssociatesMaterial.
In simpler terms, if a wall “X” is typed by wall type “Y”, and wall type “Y” has a material layer set “ABC” (a usage is not allowed on a type), then the ForLayerSet of wall “X” may (i.e. can) reuse the material layer set “ABC”.
Similar to Must MappedRepresentations come from the corresponding IFC Type? - this probably requires clarification, as material layer sets are a form of implicit geometry.
My proposal is that the word may be changed to must.
Therefore, if a type has a material layer set, any occurrences of that type which have a material layer set must reuse the layer set.
Similar argument for profile set usages.
Thoughts?