This is a reusable definition. Requiredness is relative to this structure; its use and conditions in a message remain on that message's page. Version links browse the other dictionary, not an assumed equivalent component.
Used to restrict updates/request to a list of specific CompID/SubID/LocationID/DeskID combinations.
If not present request applies to all applicable available counterparties. EG Unless one sell side broker was a customer of another you would not expect to see information about other brokers, similarly one fund manager etc.