What difference?   

Who is Pre-G's Additional Members?
  Pre-G's Additional Members are supporting members of Pre-G to fullfill the line up when they promote as larger group (In 9 members album/single). Easiest to explain is they like AOA Black's Y who will join only when AOA promote as band group. But when Pre-G promote as 7 members girl group, you still see them as main actress in the MV or back up dancer of them.

Who is Pre-Gss?
  Pre-Gss is like a back line up or younger sister of Pre-G. They will got at least a track list in studio album of Pre-G when they promote as full album (not in debut album) and they will join some of Pre-G stage when promote that album on music program and also shows during that album promotion. But when they promote as single or mini album they won't join. Well, SOME of them MAY got opportunity to be Pre-G's back up dancer or cameo in MV.

So, What difference between them?
  Easiest way to said is you will always see Pre-G's Additional Members in every single even they didn't got any line in that single. While Pre-Gss won't appear as much as the first group. They mostly appear when Pre-G promote as 9 members single or as member of Sub-unit of the whole Pre-G project.

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