What are Upper Voices?
Upper Voices are people who sing in the treble clef, traditionally called sopranos or altos. It also includes countertenors (tenor/bass singers who have a strong and developed falsetto).
Often, vocal ranges are tied to gender, but this is a false relation. Your vocal range has everything to do with physiology (the size of your vocal folds and larynx), and nothing to do with your gender.
Am I an Upper Voice?
The range of a choral soprano spans the treble clef, from middle-C to G at the top of the staff. The range of an alto is lower, from the G below middle-C to D at the top of the staff. See this chart of the choral vocal ranges on a staff.
If you sing comfortably within the soprano or alto range, you may be an Upper Voice!
The tenor and alto ranges overlap, so the difference is in the tone quality (timbre): when a tenor sings an F or G in the middle of the treble clef staff, they are usually singing near the top of their vocal range, and it sounds like they are singing high notes (the same way a soprano sounds like they are singing high notes at the top of their range). An alto, on the other hand, has a very different sound when singing those same notes because they will be in the middle of their range, which is a very comfortable place to sing.
Often, tenors have a ‘brighter’ sound, while altos have a ‘warmer’ sound when they sing the same notes. In this choir, we are looking for the ‘warm, comfortable range’ sound rather than the ‘singing high notes’ sound.