TT #226 · Parshas Tazria · Aliyah 7 · Passuk 42
English: But if a white-reddish lesion appears on the bald area or the front bald area, it is an eruptive tzaraas on his bald area or front bald area.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:42
Torah Temimah cites Sifra (Toras Kohanim) for the distinction between karachas and gabachas by direction from the crown.
Sifra — Vayikra 13, Midrash Sefer
English: The Sifra defines karachas as the bald area that slopes backward from the crown.
The Sifra defines the anatomical area called karachas.
Karachas is the posterior bald area, where the head slopes down from the crown toward the back.