Beneficial additives alter friction characteristics
While there are many different types of ATF on the market, a few aftermarket chemical companies have come up with an idea to save repair shops and vehicle owners money. What they've done is develop additives that mix with standard Dexron III/Mercon, to alter the friction characteristics to match the other types of fluid.
One of the most common of these additives is the HFM additive. When added to Dexron III/Mercon, it alters the friction characteristics enough to allow you to use it in any transmission that requires HFM fluids.
This probably won't affect you for adding a quart of ATF; chances are you won't be able to get this additive anyway. They're usually only available through professional sources. If you need to add a quart of ATF to your transmission, you're probably better off using the factory recommended ATF.
But if you have your transmission serviced and the fluid replaced, the repair shop may use one of the HFM additives along with Dexron III/Mercon in your trans. That's okay; these additives work just fine for altering the fluid's friction characteristics.
However, if your transmission is still within its factory warranty, check with the dealer or a factory representative before allowing anything in your transmission besides the factory recommended ATF. Other additives or oils could affect your warranty.