What are the connotations of "Tahor Hu"?
Rashi: It is Tahor from Tum'as Nesakin, but subject to Tum'as Nega'im of the skin; absolute Tum'ah is through white hairs, Michyah or spreading.
What is the practical difference between this case and that of Nesakim?
Ra'avad (on Toras Kohanim, Ramban): Nesek is when only a patch falls out; hair surrounds it. Most Meforshim say that it is a Nega without any change in the skin's color. Here, the hair fell out from the entire front or back of the head; Nega'im apply just like on regular skin.
Ramban #2 (according to his understanding of Rashi): Nesek is a known Nega (the Torah did not explain what is its appearance).
Ramban #3: The current case speaks where the Siman Tum'ah appears on the bald patch of regular skin. 1
RS"R Hirsh (from R. Shimshon on Mishnah 10:10): Nesek is when hair falls out temporarily; here it fell out permanently. The Ramban supports this; Nituk implies detached, and Meritah implies a fixed change in the skin. 2
However, the Ramban himself (See Pasuk 29-30 DH 've'ha'Parshah'), writes in conclusion that the current case discusses where the bald area is not surrounded by hair on all four sides (though it is not clear why - EC).
We must say that it is recognized now that hair will never grow back. If not, the Kohen would not know how to rule about it! (PF)
QUESTIONS ON RASHI
Rashi writes that Karachas and Gabachas are Tamei through white hairs. This is unlike a Mishnah (Nega'im 4:3) and Toras Kohanim (11:1)!
Mizrachi: In the Beraisa of R. Yishmael at the beginning of Toras Kohanim, it says that Karachas and Gabachas are Tahor only from Tum'as Nesakim, but they have Tum'as Ohr Basar (so white hairs are a Siman of Tum'ah). 1
Why he does not address that below (11:1), Toras Kohanim explicitly says that white hairs are not a Siman Tum'ah in Karachas and Gabachas?! (PF)