At shows judged on the group system all the Best of Breeds(BOBs) go forward to compete in the group(providing they are unbeaten). The group winners, ie the dogs which take Group 1 or Best in Group, go forward to compete for Best in Show.
The judge often short lists in the group from maybe 20 dogs down to 6 or 8 then they pick out their Group 1(Group Winner), Group 2(or reserve in group), Group 3 and finally Group 4. This happens for all of the groups we have in this country, Gundog, Pastoral, Working, Hound, Terrier, Toy and Utility so at a show judged on the group system only 7 dogs would compete for Best In SHow(BIS), providing they all attend!
Aty each different show they group judge is different so the placings wil also be different, although the 'top dogs' often get placed at all the shows, not always of course but often!
Hope this answers your question and that I haven't just confused you even more :D