mostly for 'live' chats, as in IRC..

BBS=Be back soon..or BBL=Be back later
RTI=real time interruption

RT & VT are conventions refering to Real Time..like what calls you back to work just as you are getting involved throughly in VT..Virtual Time...which is the time you spend at the Mudcat Tavern or campfire..(and ain't it easy to get 'em confused!?)