Stem-mastering is a technique derived from stem mixing. Just as in stem-mixing, the individual audio tracks are grouped together, to allow for independent control and signal processing of each stem, and can be manipulated independently from each other. Most of the mastering engineers require music producers to have at least -3db headroom at each individual track before starting stem mastering process. The reason for this is to leave more space in the mix to make the mastered version sound cleaner and louder. Even though it is not commonly practiced by mastering studios, it does have its proponents.
