I noticed that you can set the mode to +r, what does it do?

Well that depends on what server you are on. mIRC doesn't make up the modes, the makers of IRC servers make them up, and therefore they have different effects/meanings on different servers. A common meaning for mode +r is that whoever is using it is using a registered nickname, or that the channel that is using it is a registered channel (with services: NickServ, ChanServ, etc).