There are several different ATF formulations, but they are all basically hydraulic fluid.
I would check with some other sources, but ATF is pretty commonly used in manual trannys, especially in aging trannys that need a little extra help. It is debatable if they allow more wear to internal parts, but I'm not a manual tranny expert. I just know that ATF sure did solve a couple synchro problems in my BMW/Getrag five-speed, but it did make it a little noisier.