Overview#Sid need some context Session ID) is defined in OpenID Connect Front-Channel Logout as an OPTIONAL String identifier for a Session.
Sid represents a Session of a User-agent or device for a logged-in End-User at a Relying Party. Different Sid values are used to identify distinct sessions at an OpenID Connect Provider. The sid value need only be unique in the context of a particular issuer. the sid contents are opaque to the Relying Party. sid syntax is the same as an OAuth 2.0 Client_id.SID for Security Identifier