This is intended to be used when recording using "Attribute Record Mode: Active" - it's a way of telling the console to record the attributes of a light that isn't actually on.
M is a special level between 0% and 1% that means that a channel basically is off but should still be considered (for Move Fades, Attribute recording etc.) . Having a level of 0% normally means that a channel isn't included.
The M(ark) indication is the same for all channels. They don't have to be devices.
