The reason how we distinguish the theme of soprano sound from other instrument sound is easy. As when we play the piano and listen to the sound, the highest sound is soprano. So when we play the piano with the right hand, the finger for the end of right is soprano. So it is supposed to fit to make soprano sound distinguished from other part of sound. Also the highest voice is soprano.

The highest sound can be suddenly low when we consider this high sound as not the soprano note but the highest sound as the flow.

This point of view is not always right for classical music. The problem is more profound.

Don't forget diapasone is not tessitura.
I'd distinguish profiles: high voices and low voices.

For musical instruments we should notice the fact alike: some instruments play high notes ok, some play it with force; that means the timbre will show how suitable this note is.

By the way, Soprano's repertoire is often sang by tenor, isn't it?