Retail means you are physically there. A cashier, a kiosk, or a window prints or records the bet while you stand in a licensed building. The app on your phone can be legal in the same state and still not be retail.
In Nevada and Atlantic City, retail usually looks like a race-and-sports room: boards, seats, ticket writers. In Colorado or a lottery state, retail may be a terminal at the end of a bar. Both count. They do not feel the same, which is why we tag venue types.
Some states are retail-only. Mississippi, Washington tribal floors, Deadwood — you do not get a statewide app from the couch. Other states are online-only. Tennessee will sell you a legal bet and still not give you a casino book to sit in.
Age is 21. ID is not a suggestion. If a compact says the book is on tribal land, the driveway is part of the rules.