The csa_referring_call_lists array is the list of COMPOUND requests, identified by session ID, slot ID, and sequence ID. These are requests that the client previously sent to the server. These previous requests created state that some operation(s) in the same CB_COMPOUND as the csa_referring_call_lists are identifying. A session ID is included because leased state is tied to a client ID, and a client ID can have multiple sessions. See Section 2.10.6.3.
A to-do comment in fs/nfsd/nfs4callback.c suggests that NFSD should support referring call lists, though it does not provide use cases.
From RFC 8881 Section 20.9.3:
A to-do comment in fs/nfsd/nfs4callback.c suggests that NFSD should support referring call lists, though it does not provide use cases.