The ISO AggregateInformation object provides a powerful mechanism for grouping multiple metadata records into sets based on a number of relationships described by the AssociationType and InitiativeTypeCodes. Note that the sets are identified by MD_Identifiers which suggests that they could have metadata records of their own.
The MD_AggregateInformation object includes CI_Citation, and MD_Identifier fields for describing, identifying and referencing related datasets. The object also includes associationType and intitiativeType codelists, which provide context regarding how the datasets are related.