Any note may be a pitch note; for orchestras custom has settled upon a' in the treble clef, for organs and pianos in Great Britain c 2, and for modern brass instruments b flat'.
In addition to this Guido is generally credited with the introduction of the F clef.
In fact you can change clef within a part.
A part in alto clef, for instance, does not necessarily demand an alto clef, for instance, does not necessarily demand an alto trombone.
The viola part is for viola notated using the alto clef.