KDE System Settings > Startup and Shutdown > Session Management > On Login

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KDE Help Center - Session Management
On Login You may choose one of three options on what should happen
when you log into KDE:
Restore previous session If this option is
checked, KDE will save your current session's state when you logout.
KDE will restore your session on the next login, so you can continue
to work with a desktop just like you left it.
Restore manually saved
session Instead of restoring KDE to the state it was when you logged
out last, it will be restored to a specific state that you have saved
manually. If this option is checked, the start menu offers an
additional item Leave → Save Session.
Start with an empty session If
you choose this option, KDE will never restore sessions that it has
saved.
KDE UserBase System Settings/Startup and Shutdown : http://userbase.kde.org/System_Settings/Startup_and_Shutdown