CH is generally refered to in the gaming community as "Channel". (As in the multiple servers one can play)
This being said, it is often used to express when a group of players want to switch from one "channel" or "server" to the next.
Maple Story:
Gamer 1: There are too many people grinding the teddy bears here, let's switch to ch3.
Shaiya:
Gamer 1: The Lailah ch is busy right now, lets grind on the Teos ch.
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