What is the factual difference between a Nega in the location of flesh and a Nega in the location of hair?
Ramban #1 (according to his interpretation of Rashi): The sign of Tum'ah on the latter is two yellow hairs that grow on black hair that is still intact. 3
Ramban #2: The latter, like the former, requires a patch of white skin 4 with one of the four appearances (Se'es, Baheres and their Toldos), only instead of two white hairs, the Siman Tum'ah of the latter is two yellow hairs.
Ramban #3 (based on Tosefta 1:4): The latter, like the former, requires skin without hair, but they are not limited to the four appearances of white. It can be a white patch on black (skin of the head) or a black patch on white. Ra'avad (Hilchos Tum'as Tzara'as 8:1) - the Tosefta implies that a changed appearance is needed, but Toras Kohanim implies that it is not needed.
What are the connotations of "u'Mar'eh ha'Nesek Amok min ha'Or"?
Ramban #1: It refers to the yellow (bright-colored) hairs that look deeper than the skin. 1
Ramban: It refers to the Nesek that turned white after the black hair fell out. 2
What is the meaning of "Nesek Hu"?