I think there's an older European notation that puts it like that. In standard American notation, it'sI think I'll just remember that a soprano's high C is C3 and count down or up from there. But only when I'm forced to at gunpoint.
C6: soprano high C
C5: tenor high C
C4: middle C
C3: low C (low range of tenor)
C2: deep C (low range of basso)
the number changes at C, so B5 is just before C6 (ex: my range in peak vocal condition was around D#2-Bb4, my good notes were from around F2-Ab4)