Normally, when you are printing an edition, you are striving to make each print in the edition as close to identical as possible. Not always though.
I am not talking about monotypes or monoprints here. A monotype is created from a completely blank plate by drawing / painting with ink and making one or more impressions on a sheet of paper or other substrate. Each monotype is completely unique: an edition of 1. A monoprint is similar, but the plate (or plates) will have some information on them, so the drawing is augmenting the normal printed process. So you might have a normally etched plate and a blank plate where the artist draws / paints that combine to make an image. Again, each one is going to be pretty much completely unique: an edition of 1.
But sometimes, there are more subtle differences. Most of the image information is contained on the plates, but the artist is manipulating the printing in some way that is unique and not completely reproducible proof to proof. Perhaps they are intentionally leaving plate tone. Perhaps they are inking the plate with multiple colors simultaneously in an organic shape. They may be adding color by hand after the printing is complete. The result is a set of prints that are very similar, but not quite the same. An edition that varies, if you will. The technical term for this is a Variable Edition or Edition Variable.
Sometimes an edition of this sort will not be numbered any differently than normal. At other times, you may see the nomenclature 1/20 EV or 1/20 VE. Like so many things, there really are no hard and fast rules, so each artist or publisher will have their own way of doing things.
I recently completed a variable edition of a new print, Grade Crossing. This was a four color woodcut, each plate cut on MDF. While printing the first color, after rolling up the plate, I used various techniques to push the ink around on the plate and add some texture to what would have otherwise been a flat. Subsequent plates were printed normally. The effect is subtle, but noticeable, especially when you see several of the proofs side by side.