Also, could be a set made by little Gremlins.
Or a set kept on a low table.
Actually I've seen people refer to sets or albums that only contain a year,
no mint marks as a short set.
For example too is the Whitman Large Cent Album for year only, no varieties as a short set. Also, there are numerous Albums that do not contain places of Proofs.
I would assume that any Album, Folder or set of coins that could be completed without certain types of varieties as a short set.