$Api

$Api.Session_ID and GETSESSIONID() return the same value, an identifier for the current session in the current context. This context varies depending on where the global variable or function is evaluated. For example, if you use either in a custom formula field and that field is displayed on a standard page layout in Salesforce Classic, the referenced session is a basic Salesforce session. That same field (or the underlying variable or formula result), when used in a Visualforce page, references a Visualforce session instead.

Session contexts are based on the domain of the request. That is, the session context changes whenever you cross a hostname boundary, such as from .salesforce.com to .vf.force.com or .lightning.force.com.

Session identifiers from different contexts, and the sessions themselves, are different. When you transition between contexts, the old session is replaced by the new one, and the old session is no longer valid. The session ID also changes at this time.

Normally Salesforce transparently handles session hand-off between contexts, but if you’re passing the session ID around yourself, you might need to re-access $Api.Session_ID or GETSESSIONID() from the new context to ensure a valid session ID.

Not all sessions are created equal. In particular, sessions obtained in a Lightning Experience context have reduced privileges and don't have API access. You can't use these session IDs to make API calls. {!$Api.Session_ID} isn’t generated for guest users.