Consider the following explanation in the Managing Sessions page in the QtC manual:

"For example, if you work on a project and need to switch to another project for a while, you can save your workspace as a session and then close the project and all the files in it. Everything that you open after saving the session becomes a part of a new session."

Sessions are saved automatically. To "save" a session, the user either should switch to another session so that the contents of the current session are kept, or the user should clone the current session, most likely the default session, with a proper name and continue changing the current session.

The documentation doesn't mention none of the above possibilities. Since there is not a Save button for a session, the quoted explanation is more confusing than helpful. IMHO, Managing Sessions page requires some improvement.