You can use either.
Example 1
Asynchronous Timer: Frames = 6
Hitbox
Asynchronous Timer: Frames = 9
Terminate Collisions
With this example, the hitbox activates on Frame 6 (+1), and terminates on Frame 9 (+1).
Example 2
Asynchronous Timer: Frames = 6
Hitbox
Synchronous Timer: Frames = 3
Terminate Collisions
With this example, the hitbox activates on Frame 6 (+1), and terminates 3 Frames later.
The (+1) marker is because Brawl activates the commands one Frame after they were called.