From OpenKM Documentation
In Relation Types view, an administrator can define the different relations that can be set between folders, documents, mails and records.
In relation types view, you can view the relation types that have been defined:
Creating a new relation type
- Bidirectional: This relation indicates the equivalence between the objects that belong to it.
- Parent / Child: Defines a belonging relationship.
- Many to many: This relation defines a group relationship.