So was it ever determined when someone with this credit is listed as a Show Star, and when they're a Guest Star? I see it both ways on different shows.
Probably it could be determined that there are reasons to do it differently, such as "Well, if it's font size X", or "It's placed after the main crew while the main stars are placed before the main crew". But that seems kind of complicate. Why not just "If it says 'Also Starring', it's X"?