It depends. Some companies choose to stock shelves themselves in order to make sure their products are properly stocked. Coke/Pepsi and their subsidiaries are this way. It ensures that old out of date product isn’t getting shoved to the back of a shelf, allows them to keep a better track on refills (thus an estimate of sales), so on and so forth. In other cases, the store sometimes requires it to alleviate their own stocking burdens. The vendors are also generally the ones who set up the sale advertisement displays (end-cap or stand-alone). It tends to be a win-win because the vendor gets to advertise with a large display and the store may sell more product (and doesn’t have to set the crap up or take it down). Though, anything can be anything from store to store, so these aren’t “rules”.