Nolo's website says the following about an artist's right to assign their work versus an artist licensing their work: "In a contract for the production or sale of your artwork, you can do one of two things: license your artwork or assign your work. An assignment is like selling your home- you usually give up all ownership rights to your art. A license is akin to renting your home- you temporarily transfer your rights but you retain ownership of the art.
If you get the rights back to your artwork, you are most likely dealing with a license. This return of ownership is usually referred to as a 'reversion' or 'reversionary rights' and is found in most license agreements."