The below is from the Preface to the 2012 Official Playing Rules and Casebook of the National Football League (Copyright © 2012 by the National Football League.).
This edition of the Official Playing Rules of the National Football League contains all current rules governing the playing of professional football that are in effect for the 2011 NFL season. Member clubs of the League may amend the rules from time to time, pursuant to the applicable voting procedures of the NFL Constitution and Bylaws.
Any intra-League dispute or call for interpretation in connection with these rules will be decided by the Commissioner of the League, whose ruling will be final.
Because interconference games are played throughout the preseason, regular season, and postseason in the NFL, all rules contained in this book apply uniformly to both the American and National Football Conferences.
At many places in the text there are approved rulings which serve to supplement and illustrate the basic language of the rules. Each is headed by an abbreviation, followed by a number (e.g. “A.R. 3.20”). The letter “A” in an approved ruling indicates the team that puts the ball in play, and its opponents are designated by the letter “B.” Whenever a team is in possession of the ball, it is the offense, and at such time its opponent is the defense. Yard lines and players are indicated by numerals. Thus, for example: “A.R. 50.1 Third-and-10 on A30. During a run prior to an intended pass by quarterback A1, defensive player B1 holds flanker A2 on the A45…”
Where the word “illegal” appears in this rule book, it is an institutional term of art pertaining strictly to actions that violate NFL playing rules. It is not meant to connote illegality under any public law or the rules or regulations of any other organization.
The word “flagrant,” when used here to describe an action by a player, is meant to indicate the degree of a violation of the rules—usually a personal foul or unnecessary roughness—is extremely objectionable and conspicuous. “Flagrant” in these rules does not necessarily imply malice on the part of the fouling player or an intention to injure an opponent.