In short: JIP
Introduced in Operation Flashpoint: Elite.
This feature allows you to join in a multi player match/game/mission while it is running, you don't have to wait for the next mission as you do in Operation Flashpoint.
To disable the JIP on the server, admin simply disables any AI (playable slots) on the role selection screen. However, if respawn is enabled in the mission, it's still possible to get into the mission.