mode since description
a 0.3.0 User is away.
+ b 20 User blocks private messages and notices.
B 20 User is flagged as a "bot".
c 17 IRC operator wants to receive connect/disconnect NOTICEs.
C 19 Only users that share a channel are allowed to send messages.
R 19 Only registered users are allowed to join this channel.
s 0.9.0 Channel is "secret".
t 0.3.0 Only ChanOps are allowed to modify the channel topic.
+ V 20 Channel doesn't allow invites.
z 16 Only users connected via SSL are allowed to join the channel.
III. Channel User Modes