OData-Aggr, section. 3.1.3.6 states that services can define custom aggregation methods, and the annotation term property Aggregation.ApplySupportedDefaults/CustomAggregationMethods is used to advertise them.

But this leaves no room for explaining the behaviour of a custom aggregation method. Could this be modelled as a function tagged with a new term Aggregation.IsCustomAggregationMethod (PR #145)? The one from example 13 in OData-Aggr would then be modelled as

<Schema Name="Custom">
 <Function Name="concat">
  <Annotation Term="Aggregation.IsCustomAggregationMethod"/>
  <Annotation Term="Core.Description" String="Concatenates distinct string values separated by commas"/>
  <Parameter Name="InputSet" Type="Collection(Edm.String)"/>
  <ReturnType Type="Edm.String"/>
 </Function>
</Schema>