USSSA refers to an illegal substitution as "Illegal Player."
Sec 7. ILLEGAL PLAYER is a player who enters the line–up without the right to an offensive or defensive position. Examples of an Illegal player(s), but not limited to, are:
A. Starter and/or substitute who re-enters in an incorrect batting position.
B. Starter and/or substitute who re-enters a second time.
C. Using a player to pitch who was removed from the pitching position because of Rule 4 Sec 8 A "Penalty".
The use of an illegal player is handled as a protest that can be made at any time, while the player is in the game.
A player will not violate the illegal player provision until he/she enters the game and one (1) pitch is thrown. Any action before one pitch is thrown is correctable.
PENALTY: An illegal player violation results in the immediate ejection of the illegal player and his/her coach. In addition, the following penalties will apply:
A. If the illegal player has completed a turn at bat and before the next pitch, the illegal player is called out. Any advance by any runner as a result of the illegal player becoming a batter-runner is nullified. Any additional outs recorded on the play stand.
B. If the illegal player has made a defensive play and before the next pitch (or the defensive team or the Umpires have left the field), the offensive team has the option of 1) the result of the play or 2) replaying the last pitch.
NOTE: The illegal player can be legally replaced by any eligible substitute.