So far I have been allowed almost complete experimental freedom.
On rare occassion I get simple specifics like "A bookmark type!", or "Realistic", "Specific color included".
Sometimes I stalk people's galleries, profiles and favourites to see if they have any obvious likes, favourite colors or styles or something to add into the picture.
Though I tend to be very specific when commissioning people haha Almost to the point they are probably scared of messing up XD I try to let them know it's just my preferred idea but they can tweak it to fit their inspiration or style. Sometimes their idea looks cooler than what I wanted!