What are substitutions in Baseball?

Section 4: Players/Officials

Each team plays nine defensive players: pitcher, catcher, four infielders (first, second, third base, shortstop), and three outfielders (left, center, right).
The batting order lists the players who will bat and must be followed in sequence.

The NCAA permits a designated hitter (DH) who bats in place of one defensive player, typically the pitcher.
The NCAA also allows a player to serve as both the pitcher and the designated hitter in the same game; if the player is removed from one role, specific rules govern whether the other role continues.

Substitutes enter by being reported to the plate umpire and take the replaced player's spot in the batting order.
No re-entry: Unlike the high-school game, NCAA baseball does not permit a player who has been removed to return. Once a starter or substitute leaves the game, that player is done.
Courtesy runner: The rules permit a courtesy runner for the pitcher and the catcher to keep the game moving without using a substitution.

Umpires: Games are worked by a crew — a plate umpire plus one to three base umpires (two-, three-, or four-umpire crews). The plate umpire calls balls and strikes and has general charge of the game; base umpires rule on plays on the bases and in the outfield.
Official scorer: Keeps the official record — runs, hits, errors, and the line score — and rules on hits versus errors and earned versus unearned runs.
Head coach: Responsible for the team's conduct and is the point of contact with the umpires for lineup changes, conferences, and appeals.

Each team plays nine defensive players: pitcher, catcher, four infielders (first, second, third base, shortstop), and three outfielders (left, center, right).
The batting order lists the players who will bat and must be followed in sequence.

The NCAA permits a designated hitter (DH) who bats in place of one defensive player, typically the pitcher.
The NCAA also allows a player to serve as both the pitcher and the designated hitter in the same game; if the player is removed from one role, specific rules govern whether the other role continues.

Substitutes enter by being reported to the plate umpire and take the replaced player's spot in the batting order.
No re-entry: Unlike the high-school game, NCAA baseball does not permit a player who has been removed to return. Once a starter or substitute leaves the game, that player is done.
Courtesy runner: The rules permit a courtesy runner for the pitcher and the catcher to keep the game moving without using a substitution.

Umpires: Games are worked by a crew — a plate umpire plus one to three base umpires (two-, three-, or four-umpire crews). The plate umpire calls balls and strikes and has general charge of the game; base umpires rule on plays on the bases and in the outfield.
Official scorer: Keeps the official record — runs, hits, errors, and the line score — and rules on hits versus errors and earned versus unearned runs.
Head coach: Responsible for the team's conduct and is the point of contact with the umpires for lineup changes, conferences, and appeals.