It's a small sketch by the signature, usually placed for a nominal fee...
On a print of a watercolor spring farm scene, I have a sketch of two geese, done in pencil, cost $25. A real personal touch and very interesting. On a print of a pencil drawing of a building I designed, the artist placed an enlarged detail of the building. (No charge because I helped him with something else.) Stuff like that. Maybe for a 100 or a 3000 a sketch of the 'flash' or something similar, I've always left it up to the artist.
Thanks for asking!