There is e.g. shop=mall encompassing all as additional tag to a building marked as e.g. retail or commercial to note a few which then could be called the name the mall was given. Then it's up to the mapper to choose creating the individual points of interest (POI) and place them as close as possible within the building outline as to where a shop can be physically found to include if you'd include their mail addresses add the tag addr:floor = n (so happen to do that today). These POI should not be put on any building line, you'll be told to extract them again and move them in a free space. Certainly I'd not tag the whole building with a name like Tesco if Tesco is just a part.
In the extreme you could map each individual shop in a building and then bind them together in a relation but do not remember to have ever seen this done.done.
@ Tzorn, over here there's the big Conad chain which runs sub businesses such as the branded Parafarmacia Conad. These are put in a separate POI within the Conad area of operation.
