If you mean cashier backup. I always worked stores where SD was immediately in front of the lanes. So all 10 items or less purchases were sent there if the lanes were backed up. Food Ave never had to help with cashier backup. We needed the FA register for food purchases. Of course, I would get people bringing stuff and asking if they could check out there. I was always polite and said Yes, as long as it wasn't any chemicals.
From the perspective of help coming to FA, in my experience, don't expect it often.
At some higher volume FA's, you will have double coverage. I remember at one store we had 3-4 TM coverage on the heavier shopped weekends. Then at lower volume FA's, you will be alone. Sometimes if there was FA trained TMs in the store, we would back them up during lunch or having to dump grease or help prep when they are taking orders.
At my former high volume store, there was one holiday weekend where it was just down right insane, and my FA just lost 3 people real quick (new job/store theft/quit Target as 2nd employment). And my FATL decided at noon (which was my lunch time) to take her lunch then go work on office work for the next 2 hours leaving me solo during a rush. I was hitting the assistance button every 5 minutes for an hour before they sent a cashier with no FA experience over. I ended up clocking out for lunch 2 minutes before compliance.
Then at another store, every July 4th the entire city comes to my former lower volume Target parking lot to view the city's fireworks. This FoodAve was always single coverage. And I voiced to my ETL-GS/SL and GSTLs that we will need at least double coverage for the 4th. Their response "how bad could it be..." At 730pm, it was pretty slow and parking lot was about empty. I was really trying to get ready to close. By 800pm, the parking lot was packed and people were lined out of FA to the doors. I went through maybe 2 boxes or more of pretzels (had to microwave them to order), made a number of sandwiches, and popped batches of popcorn continuously. FA went from spotless to total devastation within minutes. My ETL-GS/SL ended up coming over at 900pm to block the end of the FoodAve line then only came over to help me finish the with the last crowd of guests. I ended up having to block FA with carts to keep people away.
It took me 2 hours to re-prep and clean Food Ave. The closing team was walking out the door when I got finished.