MASK allows you to screen out active levels in the current
part. For example, if you have created entities on levels
one to five, and want to delete only the entities on level
four, assign the level mask to 4.

You can also use the MLEV: function in the Status Window,
or the assigned Immediate Mode command to set a level 
mask.

Enter the level number you want to mask on, or enter zero
(0) to assign no level masking at all.
