Halakhah su Bega'im 1:1

And the matter of tsaraat is that one or more places on the skin of the flesh of a man become white - and these places be very white until their white resembles the white of the membrane of an egg and more than it in its whiteness. But the whole time that it is less dark in its appearance than the membrane of an egg, it is not tsaraat but rather a shiny spot (bohak) - meaning to say, a different illness that is not a type of tsaraat at all, but rather like types of rash and other types of [skin] ailments that occur with a person. There are four appearances to the tsaraat on the skin of a man's flesh: Two are primary sources (avot) - and they are the se'et and the baheret - and two are their derivatives. And that is [the meaning] of their, may their memory be blessed, saying (Mishnah Negaim 1:1), "The appearances of ailments are two which are four: Baheret [...] and its adjunct; se'et [...] and its adjunct." And the understanding of its adjunct is meaning to say, its derivative; as the expression, adjunct (sapachat) is only an expression of [being] secondary. And the Sages likened these four appearances: one to clean white wool, and that is the se'et; one to snow, and that is the baheret; one to the lime of the sanctuary, and that is the derivative of the baheret; and one to the membrane of an egg, and that is the derivative of the se'et (Mishnah Negaim 1:1). These are the impure ones and they combine, one with the other, to render impure (Mishnah Negaim 1:3). And anyone who does not recognize them through their differences and their names should only determine them by the mouth of someone who recognizes [them] (Shevuot 6a) - and as it is stated below in the laws of the commandment.