In store display means exactly what it sounds like. It's an image that will be displayed inside a store whether it be a a standee, an overhead signage...whatever. It means it's going to be used commercially and not in somebody's portfolio. It's going to be seen by every person who walks into the store/s that it's in. Also, if going to be used commercially that means it has the potential to be repurposed. So, when an image is being used commercially the general idea is to charge a commercial rate.