Middle C, for example, is an extension note on both treble and bass clefs, however is not outside the grand staff.
Soprano C and Deep C lie two ledger lines above treble and below bass respectively (as well as the grand staff).
However, since her normal tessitura is mezzo-soprano (or under Soprano C), her abilities in the whistle register would be considered her extension.
Due to the rarity of true contraltos, usually mezzo-sopranos with strong low extension are employed instead in roles such as "Ulrica" in Verdi's Un ballo in maschera.
For men, their vocal extension upwards is usually co-terminus[jargon] with his falsetto, unless they are a countertenor, barbershop tenor, or its equivalent in popular music.