A flip case (bookcase) wraps around the iPad Air (2020) / Air 4 like a book and covers the display too. That protects the screen in your bag and usually adds a card slot.
Many models include a stand function so the iPad Air (2020) / Air 4 props up for video, and a slot for one to three cards. Look for a magnetic closure that holds well and a cutout that lets you take calls without opening the cover.
If you want a slim device, a back cover suits you better. If you want cards and screen protection in one, take the flip case.
The iPad Air (2020) is the first non-Pro iPad with USB-C, Pencil 2 and Magic Keyboard — LED Liquid Retina 10.9″, A14 Bionic chip and USB-C. A good case protects the display and the aluminium back from scratches, drops and table edges, and preserves the resale value of your device.