As more department stores (even discount stores like Kohl’s) launch their own discount and off-price chains, this question of brand segments will become more important. For example, you can order from Old Navy and Banana Republic, the high and low ends of the Gap family of brands, in a single virtual cart and a single shipping charge.
However, what you can’t do is return something that you bought at a Gap store to an Old Navy, or return items from any of the brands’ respective “factory stores” or outlets to the regular store.
In the end, most of these items aren’t going back on the shelves at the Nordstrom or Rack store where you bought them, anyway: they’re going to a closeout retailer that scoops up store returns. It matters so the store knows how much money to refund you, though, requiring the different stores to have access to each others’ pricing databases.