For many instruments, such as flute or trombone, each staff usually contains a single musical line in a single voice that is read from left to right along the staff. When multiple, independent lines must be shown in a single staff, each line can be a separate voice.
One common use for showing multiple voices in a single staff is in vocal music, when the soprano and alto lines share a single staff, and the tenor and bass lines share another staff. Showing each vocal line in its own voice helps to separate the lines, making the music easier to read and the shape of each melodic line clear.
In Cubase, notes belong to voices. Voices are divided into up-stem voices and down-stem voices. Stems of notes in up-stem voices point upwards, while stems of notes in down-stem voices point downwards. However, in bars where only one voice contains notes, stem directions are automatically changed to the directions they would have if there were only one voice in the staff. By default, the first voice in the staff is up-stem.
Following most notation conventions, rests are shown in bars for all voices that have notes in the bar. If two or more voices have a rest of the same rhythmic duration at the same rhythmic position, that rest is consolidated by default: instead of showing two identical rests, only one is shown.
Voices are separated automatically according to the instrument mapping. You can change this by setting up voices and stem directions on the Instrument tab of the Layout Settings dialog, and, for drums, in the Drum Map Setup dialog.