9-2-1-b-1: "During the game, coaches, substitutes and authorized attendants in the team area shall not be on the field of play or outside the 25-yard lines without permission from the referee unless legally entering or leaving the field (Exceptions: Rules 1-2-4-f and 3-3-8-c)."
The two exceptions are for kick practice in limited sized stadiums and timing in the last two minutes of each half when the stadium clock is not the official timing device.