The automatic transmission on my 2015 Toyota Tacoma is a closed system. It doesn't even have a dipstick. You check the fluid level by parking on a flat surface and draining fluid from the pan from a screw in the bottom of the pan. Above the drain screw is a little straw that sticks up a couple inches into the pan, the top of which equals the "full" level. The engine must be just below operating temperature for volume measurement to be accurate. Checking the fluid level this way is complicated and cryptic with no failsafes. What happened to the good old dipstick? It was deemed too expensive to manufacture. Oh ya? Try buying a new transmission.
Any transmission will fail quickly if it loses its fluid - even a relatively small amount. Circulating transmission fluid provides several crucial functions, including lubrication, heat dissipation, and - most importantly - viscous coupling during gear shifts.
In the first set of diagrams below, I describe the parts of a simple, closed system analogous to a sealed transmission with three gears. The second set of diagrams show 3 modes of failure.
The goal of this exercise is to demonstrate that incomplete transfer of fluid inside a sealed transmission may cause symptoms of low fluid levels/low pressure/excess heat even though the level reads "full".
In Examples #1 and #3, the total fluid volume of the system remains unchanged (fluid level = full). In example #2, fluid volume is actually lost due to the phase change of the fluid to a gas. In all examples, no fluid escapes the case.
Repair for Example #1: Fix would involve replacing one or more valves.
Repair for Example #2: Fix would involve replacing defective or damaged internal workings of A. Fluid must be flushed and replaced. A determination of what caused the parts to stick must be made (old fluid? clogged valve body? stuck valve? electrical problems at valve or control module?)
Repair for Example #3: Replace pump and/or internal seal(s).