Описание тега sessionid

A session ID is a unique identifier that relates session data to a specific user.

In an HTTP context, it is usually stored in a cookie and sent to the server on every request. The server uses the session ID to load the data that was persisted in the session.

The lookup process on the server can be either file-based (where the session ID is used as part of the file name), or from a database (where the session ID is used as the primary key).