It is commonly associated with IRC chat,
Meaning Internet Relay Chat.
!sync is a known command for the module bs_fantasy_ext
commonly associated with Anope.
It is used to syncronize all users access in the channel.
Note: By default only Admin (&) and Owners (~) can change users access, See !acc
<~TMFKSOFT> !acc add User 5
-BOT- Synchronized userlist with accesslist.