Mastering is the final step in the process in which the final stereo mix is polished and ready to be distributed. Mastering typically involves subtle enhancements and focuses on achieving competitive loudness, consistency, tonal balance and dynamic control.

A Stereo Master is a single file and it is the final stereo mix ready to be mastered,

A Stem Master often has more control and flexibility for a Mastering engineer to address specific issues in a mix and produce a better final result. A Stem Master consists of sub-groups, usually 4-8 files of summed audio such as vocals, drums, bass, guitars or other instruments and effects.