It's undocumented, so it's not wise to use it.
From Version.txt mIRC v3.8
23.Added "me" prefix to remote definitions, eg.:
me:1:ON JOIN:/msg etc...
This limits a command definition to reacting only to
events caused by your client.
I do not see where it is said it was ommited. It's not in the help file, but I can swear I have seen it somewhere in the faq on mirc.com. It says "This flag is designed to make sure you can make events only work if somebody with exactly the same address as you use does something. This other person normally would be your 2nd instance of mIRC running from the same PC", so I think it means it is documented
