ATF is automatic Transmission Fluid.
There are different types of ATF. Your transfer case just happens to need ATF +3. Some transfer cases need Gear oil.
Here are the different types of automotive "oils".
Motor Oil (For your Engine): Usually 10W-30 Or something similar. (5W-30, 20W-50) etc... These oils are "lightweight" in the autmotive setting and are used mainly for motors. The "W" designation is the "cold" viscosity and second number is the "hot" viscosity.
You see, today chemists can figure stuff out, way back when, folks had to use "thick" oil in the summer and "thin" oil in the colder winter temperatures. Today, chemists have figured out how to do two things with one oil. (Don't ask me how it works, google it)
Gear Oil (For your gear driven units; Manual Transmissions, SOME gear driven transfer cases, Ring and Pinion / differential gears) Usually the lower viscositys are around 80W-90.
Automatic Transmission Fluid (For automatic Transmissions, SOME chain driven transfer cases): Many Types of this are available, depending on detergents. ATF +2, ATF +3, ATF +4, TYPE F, Dexron III, Dexron Mercon, etc...
Grease (for steering linkages, U-joints, bearings etc): Grease varies by heat rating. I prefer to use lucas for my driveline u-joints and marine grease for everything else.
Brake Fluid: for brake systems and hydraulic clutches. Usually DOT 3 rated.
Power steering fluid: For power steering systems. Honda uses a different formula otherwise, I think all vehicle manufacturers use the same thing.
Windshield washer fluid: For washing the windshiled. Colder climates add two shots of whiskey per quart to prevent freezing and keep your jeep smelling like whiskey.
Hope this helps.