NMI

Set to 3 different clues across 4 published puzzles, most often “Application letters” — The New York Times, Thursday, 6 January 2011.

3 letters

Every clue on record, most used first

Application letters2 times
The New York Times — Thursday, 6 January 2011
It indicates a void in some govt. recordsonce
The New York Times — Tuesday, 17 August 2010
Application form abbr.once
The New York Times — Sunday, 24 September 2006