The object is then used in draft-berger-ccamp-gmpls-segment-recovery-00 to associate LSPs in different sessions - the segment recovery LSPs and the LSP they are protecting. See the following quote from section 2.
... Each segment recovery LSP is signaled as an independent LSP. Specifically, the Sender_Template object uses the IP address of the node originating the recovery path, e.g., node C in the topology shown above, and the Session object contains the IP address of the node terminating the recovery path ...
When [FRR] isn't being used, the association between segment recovery LSPs with other LSPs is indicated using the Association object defined in [E2E-RECOVERY]. The Association object is used to associate recovery LSPs with the LSP they are protecting.
Clearly 16 bits is not enough to identify the associated LSP unless we know which session it's in.