Originally Posted by Burton

Shimano specifically documents what end-use each mtb group is suitable for on page 25 of their dealer catalogues. For example Saint is the ONLY group suitable for DH use.

The choice of a rear derailleur is most critical when a mtb styed bike is used off-road. Tourney, Altus and Acera are intended for city bikes which explains why so many people with mtb styled bikes end up with scrapped deralleurs when they start to use them off-road. Component use is about more than just a hierarchy.

I've gone out of my way to post the details in the MTB forums twice and most people apparently simply don't want to know.