There are persons in our society who, when they have learned the pitch associated with the notes on a page of music, have the innate ability to remember those pitches. For lack of a better term, we call that "perfect" pitch.

I've noted in several threads on MC of my experience with an a cappella group in which I sang in the 70's. We always hit our starting pitch based on an A440 fork, but often our conductor would have us sing down a semitone or two, based on the period of the music. For one soprano who had "perfect" pitch, this was a nightmare. For those of us with good relative pitch, it was piece of cake.