Final answer:
During a penalty shootout, no substitutions are allowed besides replacing an injured goalkeeper; the players who finish the match are the only ones permitted to take penalties.
Step-by-step explanation:
The rule regarding substitutions during a penalty shootout is that no substitutions can be made once the shootout has begun, except for an injured goalkeeper. This means that the players who end the match are the only ones eligible to take the shootout, with the exception of a goalkeeper who sustains an injury during the sequence.
No additional players may be brought on to take a penalty, and both teams must use the same number of takers if they have been reduced to less than the usual five shooters due to dismissals during the match. The match rosters are effectively 'locked in' at the point where the match ends and the shootout begins, reinforcing the fairness and equity of this tie-breaking procedure.