Stem Mastering is when the music is Mastered per group of instruments rather the Stereo Track by its own.
Stem Mastering is very common for artists wanting a detailed control on each stem, level-wise and quality-wise.
Vocals Stem is usually exported with their reverb, delay, automation and all other plug-ins inserted. Vocal Stem is usually mixed with other versions (if any) of lead vocals.
Backing Vocals Stem is usually exported with their reverb, delay, panning, automation and all other plug-ins inserted. Backing vocals can be exported as one stem with the lead vocals.
Percussions like xylophone, cymbals, darbouka, daff, rikk, triangle. Percussion Stem can be exported with the drums depending on the mix and frequency spectrum.
Drum Group mainly includes kick drum, snare, toms, cymbals, hi-hat. Drums can be exported with the bass Stem and Percussion. It is preferable though to send the Bass Stem by itself since many issues occur in this frequency range.
Bass stem includes bass guitar and other sub frequencies instruments. It is advisable to send the bass Stem by itself since many issues occur in this frequency range.
Instruments like Strings, violins, cello, guitars can form one stem and exported as a stereo group track. It is often blended with the Keys group as well.
Instruments like Keyboard, piano, organ can be exported as a single stem for mastering or exported with the strings Group depending on the mix and the frequency range.
It is completely up to you to decide the Stems Types and their content.
Any combination will work at the end, depending on the mix of each group.
For example, you can send only 3 Stems: