+i is, as far as I have seen, universally accepted as invite only. For users it is 'invisible mode'. Any network which has redefined it is not following correct protocol.
Other modes not mentioned in the RFC are subject to the ircd developers' own opinions. +r/+R being one example. +r on an Undernet channel means registered users only may join, whereas it's +R on DALnet. I would imagine this is partly why mIRC would never be able to make an arbitrary Channel Central screen with different check boxes based on the RAW reply for channel modes, as the different letters could mean different things on different networks. It can only include the basic channel modes outlined in RFC1459 which all ircds should adhere to.
Regards,
