Some transfer cases do use automatic transmission fluid in the transfer case however some transfer cases will use transfer cases require SAE 80W or SAE 80W-90 GL-5 gear lubricant.
If you have your owners manual for your vehicle it should state what type of oil the transfer case uses.
Or you can give us more information on the vehicle you're asking about and we could tell you better if your vehicle uses transmission fluid in the transfer case or not.
But most transfer cases on automatic transmission vehicles do indeed use the automatic transmission fluid in the transfer case.
Your transfer case is always working even when you're not using the 4 wheel drive so it's good to keep check on the transfer case fluid as well and change it often as in 30,000 to 40,000 miles or so to keep it working properly.