L (capital 'L'): Lock a message base
U (capital 'U'): Unlock a message base

These commands maybe used to insure that a message base is not modified while
being backed-up (but don't forget to unlock them when you're done!).

Also note: if you back up all the files (including *.lock), then the backed-up
message bases will also be locked and will need to be unlocked (i.e. *.lock
removed) before they may be used again if/when restored.