An eligible receiver is a member of the offense that is allowed to go out for a pass. There can be up to 5 eligible receivers on any given play. An eligible receiver must wear a jersey # between #’s 10-49 and 80-99. Offensive Linemen wear (#’s 50-79) and are ineligible but can report to the referee as an eligible receiver and receive passes. If I recall correctly, Mitch Unrein, a defensive tackle, was the last to pull this off for a touchdown pass for the Broncos back in 2012.
Below are a few examples of eligible receivers from formations with the Quarterback under center. The eligible receivers are shown in the Navy with Orange trim. The first shows a 3TE heavy I-formation set. The 2nd is more of a normal 3WR set. The 3rd is another 3 WR set from an Ace set and a WR on the field. The 4th is a 4WR Ace spread set.
Below are a few examples of eligible receivers from formations with the Quarterback in shotgun. The first shows a RB split out in the slot, the 2nd is more of 4-wide look with a TE still on the field. The 3rd is a 3 TE Shotgun set, and the 4th is a 5-wide receiver empty set.
The following graphic below addresses a few more specific examples.
Scenario 1 shows #80 covered up(meaning he is not eligible to go out because he is covered up, there is a man on the LOS to either side of him) by #82. In this formation, the TE on the left, the FB/RB and #82 are the only eligible receivers. This is a short yardage/goal line formation and the offense might willingly give up an eligible receiver for more prime real estate on the line of scrimmage. Scenario 1 has 4 eligible receivers. If either #80 or #82 shifted back a step (off the LOS), then there would become 5 eligible receivers.
Scenario 2 and 3 are similar to the previous versions above but they show different personnel groupings. In scenario 2, concentrate on the right side of the offense first. The TE is on the LOS, and the Z receiver is off, which ‘uncovers’ the TE. They are both eligible to go out. On the left hand side, The Y receiver is in the slot and the X receiver is on the LOS. Both of these guys are eligible to go out on pass routes.
Scenario 3 shows the same set up on the right hand side between the TE and X receiver. On the left hand side a 3rd tight end is inserted but off the LOS and then another TE is on the LOS. Both of these guys are eligible to go out on pass routes.
Scenario 4 is similar to the Unrein mentioned earlier, yet a 3rd tackle was purposely chosen to show the example of the a player #’d 50-79 has to report with the referee before the play, but is eligible to go out for pass routes as is the RTE , the RB and the pair of Fullbacks. This is another short yardage and or goal line formation and as Unrein and others have shown, the defense can fall asleep on those bigger #’s lined up in the wrong places. These are normally called tackle eligible even if the player is a defensive player.
Scenarios 2-4 have 5 eligible receivers.