This is from the NFL rule book. After reading this, I am not 100% sure either.
Reviewable Plays. The Replay System will cover the following play situations only:
(a) Plays governed by Sideline, Goal Line, End Zone, and End Line:
1. Scoring plays, including a runner breaking the plane of the goal line.
2. Pass complete/incomplete/intercepted at sideline, goal line, end zone, and end line.
3. Runner/receiver in or out of bounds.
4. Recovery of loose ball in or out of bounds.
(b) Passing plays:
1. Pass ruled complete/incomplete/intercepted in the field of play.
2. Touching of a forward pass by an ineligible receiver.
3. Touching of a forward pass by a defensive player.
4. Quarterback (Passer) forward pass or fumble.
5. Illegal forward pass beyond line of scrimmage.
6. Illegal forward pass after change of possession.
7. Forward or backward pass thrown from behind line of scrimmage.
OFFICIAL NFL PLAYING RULES 98(c) Other reviewable plays:
1. Runner ruled not down by defensive contact.
2. Runner ruled down by defensive contact when the recovery of a fumble by an opponent or a
teammate occurs in the action that happens following the fumble.
3. Ruling of incomplete pass when the recovery of a passer’s fumble by an opponent or a teammate
occurs in the action following the fumble.
4. Ruling of a loose ball out of bounds when it is recovered in the field of play by an opponent or a
teammate in the action after the ball hits the ground.
Note 1: If the ruling of down by contact or incomplete pass is changed, the ball belongs to the recovering
player at the spot of the recovery of the fumble, and any advance is nullified.
Note 2: If the Referee does not have indisputable visual evidence as to which player recovered the loose ball,
the ruling on the field will stand.
Note 3: This does not apply to complete/incomplete passes, or the ruling of forward progress.
5. Forward progress with respect to a first down.
6. Touching of a kick.
7. A field-goal or Try attempt that crosses below or above the crossbar, inside or outside the uprights
when it is lower than the top of the uprights, or touches anything.
8. Number of players on the field at the snap.
9. Illegal forward handoff.
Note: Non-reviewable plays include but are not limited to:
1. Status of the clock
2. Proper down
3. Penalty administration
4. Runner ruled down by defensive contact (not involving fumbles)
5. Forward progress not relating to first down or goal line
6. Recovery of a loose ball that does not involve a boundary line or the end zone.
7. Field-goal or Try attempts that cross above either upright without touching anything.
8. Inadvertent Whistle