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Unlike many other games, Halo PC does not require you to log in. In Counter-Strike for example, you have to enter the port and password then you can execute commands at will. In Halo you have to enter the "rcon password" before every command you make.
It stands for Remote Control. It lets you control the server by typing a series of commands. Anyone can get It, but there are passwords for every server. It shows up as a little pink line when you start It.
Initially It lets you, change maps-restart maps, kick/ban players. Change game-type, show all players In the server with the corresponding ID number.