When prompted (or when it feels like) the server sends RPL_COMMANDUSAGE:
RPL_COMMANDUSAGE <command> <context> <parameter1> <parameter2> ...
command is the verb in question.
context is a hint as to what UI element to attach the command to, or what arguments to deduce when the user enters the command as text.
channel | Any channel
nick | Any nick
nickchan | Nick in a channel
target | Nick or channel, or possibly a targetlist.
self | The user issuing the command
other | Some other context
parameters are like <type>:<name> or <type>:<name>:c, with the :c indicating the client should try to draw this from context.
label | Not actually parameter. UI text to display for this command.
sc | Subcommand; literal text to include to invoke this one.
chan | Channel name
nick | Nickname
user | Services username (which is usually a nick)
target | Nick or channel, or possibly a targetlist.
hostmask | Hostmask.
o | Other parameter
text | Long text (must be the last non-label parameter!)
So we get, assuming 2xx is the numeric:
2xx PRIVMSG target target:destination:c text:message
2xx KICK nickchan chan:channel:c nick:nick:c text:reason
2xx MODE channel chan:channel:c sc:+b label:Ban
2xx MODE nickchan chan:channel:c sc:+o nick:c :label:Give op
2xx MODE nickchan chan:channel:c sc:-o nick:c :label:Take op
2xx ZNC channel sc:detach chan:channel:c