Some servers (most notably UnderNet) control how many target nicks a user can send messages to in order to prevent spam.
(As a side note, it is supposed to limit targets to 20 over a 2 minute period, but the code seems to do something else.)
To allow ops to send more frequent messages, it implements "CPRIVMSG chan nick msg" and "CNOTICE chan nick msg" by which an op in the stated chanel can send PRIVMSGs or NOTICEs (and by extension Action, CTCP and CTCPREPLY etc.) to an unlimited number of targets.
This post is to suggest that mIRC is enhanced to support this as follows:
1. Handle CPRIVMSG and CNOTICE in 005 responses to identify support for these commands for a connection.
2. Extend all mIRC commands which send PRIVMSG and NOTICE messages to the server to send CPRIVMSG and CNOTICE instead when these are A. Supported by the server, and B. $me is an op in a common channel.