1 Flag CTF
From Halopedia, the Halo wiki
A Multi Flag CTF match generally features maps with two identical bases, each containing the team's specific flag. 1 Flag CTF isn't quite as tightly bound, and features more asymmetrical type maps, giving one base better cover and protection and the opposite base maximum exposure and much less cover. This would result in an unfair match up for the CTF Classic game type; instead the game features the teams taking turns playing offense and defense. The defensive base being the one more protected and the offensive base being the open and exposed base.
Only one team can capture the flag at a time during the game, therefore, the game is split into rounds. The players have the options to set a round limit before the game starts, but in matchmaking the game is limited to four rounds, giving each team two opportunities to capture the flag.
This game is very similar to Assault, but played the opposite way. In Assault, a bomb is taken into the enemy base, whereas in CTF the flag is stolen from the enemy base.
The offensive objective is to infiltrate the defender’s base, steal the flag, and return it to your own base to score. There are many means to how the offense can do this based on what map they’re playing on. The base may prone to better protection; however, there will be multiple ways for the offense to gain entry. Having more than one entry paves the way for the offense to fool the defense by using decoys, setting up ambushes, etc.
The job of the defending team is to defend the flag from the pursuing offensive team. If the flag is taken by a member of the opposing team, the main objective is to kill the flag carrier and return the flag to base. The defense will need to watch for multiple entries since most of the bases have more than one way to get in.
1 Flag CTF Fast
1 Flag CTF fast is similar to 1 Flag CTF, except the timer for each round is shorter, making it harder for the offense to capture the flag. Everything else is the same when compared to regular 1 Flag CTF
1 Flag CTF Pro
1 Flag CTF Pro is a variant used in Halo: Reach's Team Objective playlist.