It kinda all depends on where you've been looking.
A shell pack could be nothing more than unfinished shells with/without hardware or mounting provisions.
Or, on the other hand, a shell pack could be the drums alone. Minus heads, stands and what not.
Check out Adonis Drum Co. for some options.
A lot manufacturers of higher price range kits sell their drums as shell packs because the drummer might want to choose the mounting hardware, stands, heads, pedals etc individually. They are usually finished shells.