Rule 13 Non-Player Conduct
http://static.nfl.com/static/content/public/image/rulebook/pdfs/16_2013_Non-Player_Conduct.pdf
Section 1 Non-Player Conduct
RESTRICTED AREAS
Article 4
All team personnel must observe the zone restrictions applicable to the bench area and the border rimming the playing field. The only persons permitted within the solid six-foot white border (1-1) while play is in progress on the field are game officials. For reasons involving the safety of participating players whose actions may carry them out of bounds, officials’ unobstructed coverage of the game, and spectators’ sightlines to the field, the border rules must be observed by all coaches and players in the bench area. Violators are subject to penalty by the officials.