Look at the photo. Split the display up into 5 lines. When you
turn the screen mask on, it erases the 2nd and 3rd lines. In
this case, it would erase the "M TRUNK" (includuing the channel
activity indicator blocks) and "858.9875 NFM 4752 S-meter" lines.This leaves the 1st line containing the scan list, bank and
setting indicators, then a blank space (where the 2nd and 3rd
lines usually are) and then the 4th and 5th alpha-tag lines.
P.S. In my original photo, the data channel frequency was
display because I was holding on an inactive ID. If the ID were
detected active, the 780 would tune to the appropriate frequency
and display it in place of the data channel frequency.