While it is possible to enter mixing territory with stem mastering, that is not what it is all about. For me, this has nothing to do with changing levels, adding effects, or changing “the mix.” I offer stem mastering because I started working with indie/bedroom artists who mostly learned mixing through trial and error, books, and online videos. Many of these customers don’t have the skills or listening environment to, for example, do a good job de-essing vocals, sculpting the low end, or filtering out weird resonances from their guitar recordings. If I receive about 3-8 stems, I can make changes like these in a very transparent and nice way quickly. Sure, it is often possible to do this on a stereo track, but it will rarely sound as good.