One of the more confusing aspects of the football viewing experience for NFL fans is challenges. The basics are easy enough In any given game each team gets two challenges, and they announce their intention to use a challenge by throwing a red flag. Play is immediately stopped and the referees go to a video review to see if the challenge will be successful. If the coach gets both challenges correct, they get a third challenge. If they get one wrong, they lose a timeout and the ability to earn a third challenge. They can't use those challenges within the last two minutes of each half.
But even coaches themselves get tripped up on what, exactly, they can and cannot challenge. The fact that those rules have changed a couple of times in the last five years does not help matters-- for fans or coaching staffs across the league.
What, exactly, can an NFL coach challenge? Perhaps more relevantly, what can't they challenge?
NFL Challenge Rules
A coach can challenge all sorts of things that happen on the field. The most common occurrence is challenging whether or not a receiver caught a ball, determined by whether they maintained possession as they hit the ground or are going out of bounds. Coaches can also challenge where the officials spotted the ball on the field after a tackle or if a player was down before they fumbled. Essentially, coaches are permitted to challenge the judgement of the referees on almost any play.
We say "almost" because there is a list of circumstances in which a coach cannot throw the challenge flag. They are stuck with the judgement of the refs on that play. According to the NFL rulebook, here are the instances in which a coach cannot challenge a ruling on the field:
A team may challenge any reviewable play identified in Article 5 below, except when the on-field ruling is:
(a) a score for either team;
(b) an interception;
(c) a fumble or backward pass that is recovered by an opponent or goes out of bounds through an opponent’s end zone; or
(d) a muffed scrimmage kick recovered by the kicking team.
A team may not challenge a reviewable play:
(a) after the two-minute warning of each half;
(b) throughout any overtime period;
(c) after committing a foul that delays the next snap; and
(d) after exhausting all of its challenges or timeouts.
That just about sums it up. As long as the play is not automatically reviewable or one of the very specific instances above, coaches can throw the challenge flag and the play will be reviewed.