What is common link secret? (Verifiable Credentials using CL signature)

In Figure 11 of document of verifiable credentials data model 1.1, there is a common link secret.

However, I cannot find any explanation for common link secret in this doc. I think that it ensures legitimacy of verifiable presentation without revealing the holder’s DID.

In common link secret, what information is contained, how is it composed, what references should I refer to?