Basically just that each artist, and even within an artist's creations, might have different rules set up for different adoptables set.
A leafy adoptable might be Open, free to use however you want, credit where and when needed ect
Limited, free to use how you want but was designed for this world and shouldn't be used outside as the characteristics don't line up, usually permissions needed for ... wider usage needed
Closed, super limited, and likely even if you pay extra, you will not be able to use them as however you want, and severely limited in Rights/Copyright Transferance ect