When a photographer releases a new image as a limited edition print, test prints are made, often in different sizes or on different types of paper before an edition is created.
These pre-edition prints are known as Artist Proofs.
The number of artist proofs created is usually 10% of the size of an edition i.e. if an edition is 50 there is likely to be 5 artist proofs.
Often once the edition is sold out, these numbered or un-numbered Artist Proofs are sold for an end-of-edition price.
Being customised or rare they are often highly collectable.