Back Judge (BJ)
The back judge focuses on players on the end of the offensive and defensive lines. Lined up deep in the defensive backfield about 25 yards downfield, generally on tight end’s side, the back judge also counts defensive players and keeps track of the game clock and all television breaks.
Down Judge (DJ)
The down judge oversees the line of scrimmage, directs the chain crew and watches for offside and encroachment penalties. Lined up on the sideline opposite the press box, the down judge looks down the line of scrimmage, rules on sideline plays on one half of the field, counts the offensive players on field and informs the referee of the current down.
Field Judge (FJ)
The field judge watches the receiver that is split widest on one side of the field for illegal use of hands and blocking fouls, and the defensive back covering him for penalties. Lined up in the defensive backfield, 20 yards downfield on the same side of the field as the line judge, the field judge also watches the sidelines to determine if runners are in or out of bounds and counts the defensive players on the field.
Line Judge (LJ)
The line judge watches line of scrimmage for offside and encroachment. Lined up on the sideline opposite the down judge and looking down the line of scrimmage, the line judge counts the offensive players on the field and rules on plays near the sideline on half of the field.
Officials
The third team on the field during an NFL game, the officials are the seven men or women that enforce the rules of the game. Each official has specific responsibilities that account for all players, areas on the field and aspects of an NFL game.
Referee (R)
The referee, also known as the crew chief, is the leader of the officiating crew on and off the field. Lined up in the offensive backfield about 10–12 yards behind the line of scrimmage, the referee maintains the pace of the game, signals all fouls and counts the offensive players on the field. The referee is differentiated from the rest of the officials by wearing a white cap, is the final authority on all on-field rulings and consults with the Officiating Department in New York on replay reviews.
Side Judge (SJ)
The side judge serves as a backup for the official clock operator and serves as the primary timekeeper if clock malfunctions. Lined up in the defensive backfield, 20 yards downfield on the same side of the field as head linesman, the side judge counts the defensive players on field and signals the referee when time expires at end of each quarter.
Umpire (U)
The umpire maintains control at the line of scrimmage by watching for holding and blocking infractions. Lined up in the offensive backfield about 10–12 yards behind the line of scrimmage opposite the referee, the umpire also reviews players’ equipment, counts the number of offensive players on the field and marks off penalties.
Source: NFL.com