Lenz Digital Plus SET101 - 5.1.5 Switching the light function of the locomotive decoder:

18 First Steps SET101
5.1.5 Switching the light function of the locomotive decoder:
Action
Display
Explanation
Use the '0' key to switch function 0 (usually
the direction-dependent head light of the
locomotive) on and off. If this function is
enabled, this is shown in the bottom line of
the display.
This was a summary of how to control a locomotive using the
SET101.