CID == Connection ID (duh :P)
Basically, this switch would allow mIRC to direct events to the originating server (or CID), and to any window (chat, status, etc) that also has that CID.
For instance, you've got a custom notice event that echoes to the active window, and someone sends you a notice from a different server, as it is, it would echo into the active window, but you actually want it to echo to the server the notice originated from (at the very least, the last active window for that server connection, or to any further specified).
However, this does seem to be somewhat tricky to implement, as the notice raws don't actually tell what server the notice is coming from.