it is in the Game Operations Manual, which is separate from the NFL rulebook. from another thread:
The rulebook covers the game as it is played on the field. The game operations manual covers off-the-field issues. The chief difference is that field officials enforce the rules contained in the rulebook, while officials in the NFL's front office enforce the rules in the game operations manual.
All major sports that I know of operate this way. For example, major league baseball has rules about pre-game fielding practice and batting practice and many other issues that are not in the official rulebook.
There's also another completely different set of NFL rules pertaining to what a team can and cannot do between games and in the off-season.
There are reasons for having different sets of rules in different documents. The official rulebooks applies primarily to players and, to a lesser extent, to coaches in the course of the game.
The game operations manual applies primarily to ancillary personnel (although, as in the Belichick case, the coaching staff may also be involved).
The third set of rules applies primarily to front office personnel such as the general manager, player personnel director, et al.
