It's in relation to Dreamcast stuff. Any CDs for that platform need to have two sessions to perform correctly.
I was looking at Bleemcast in particular, where an emulator and game can co-exist on the same disc if you burn a PSX image first, leave an open session on it, and then burn the other session (with the Bleemcast binary) on it.
For the record, I tried Alcohol and that seems to do the trick. It may be naff when it comes to dumping CDs with pregap data in them but then I can just see this as a good burning tool instead. Of course, if there *are* any other programs out there which do the same thing, then it'd be interesting to know just in case any prove that Alcohol isn't all that important for anything at all. Thanks!
As for Nero, sure, it can multisession, but only with new discs you make/burn yourself, not with existing images.