Stems can refer to two things in music, relating to music notation and production.
Notation
Stems are the lines which extend from the notehead. Stems may point up or down. Different-facing stems indicate the voice for polyphonic music written on the same staff. For single-note melodies, the stems usually point down for notes on the middle line or higher, and up for those below. In music, the word texture is often used in a rather vague way in reference to the overall sound of a piece of music. ... In musical notation, the staff or stave is a set of five horizontal lines on which note symbols are placed to indicate pitch and rhythm. ...
Production
Stems are the individual components of a mix, separately saved (usually to disc) for the purposes of use in a remix. Audio mixing is used in sound recording, audio editing and sound systems to balance the relative volume and frequency content of a number of sound sources. ... A remix is an alternate mix of a song different from the original version, made using the techniques of audio editing. ...
But if you position 2 or more notes that occupy different positions vertically on the stave, but are part of a chord.
I'll further echo the request to be able to beam notes of a chord in opposite stem directions.
The rule would be that if some notes were already beamed, and other notes with stems in the opposite direction were beamed, then only those notes get beamed in the opposite direction.
Used to indicate that the notes of a certain chord are to be played quickly one after another (usually from lowest to highest) instead of at the same moment.
Thus, a quarter note in cut time is only half a beat long, and a measure has only two beats.
It is notated by a strong diagonal bar across the notestem, or a detached bar for a set of notes (or stemless notes).