In this System Settings module, you can configure Plasma's session manager and logout settings.
Desktop Session Settings
Desktop Session allows you to choose how KDE should look when you log in:
Restore the previous session
Restore manually saved session
Start with an empty session
Information
To manually save a session, you must first select the Restore manually saved session option and Apply. Then, when you use the KDE Menu -> Leave you will find that you have a new option - Save Session.