Sforno on Leviticus 14:54/ספורנו על ויקרא י״ד:נ״ד

1זאת התורה, if someone wants to qualify to issue religious rulings concerning the subject of negai-im he must first of all be able to distinguish between two different types of such afflictions although they both belong to basically the same category. This is the meaning of Deuteronomy 17,8 בין נגע לנגע, “between one kind of nega and another kind of nega.”

2לכל נגע הצרעת ולנתק. Even though both these afflictions are known as a skin affliction on the skin of a human being they are distinctly different from one another. The נתק is not judged by appearances but by the behaviour of the hair surrounding it, whereas the ordinary nega tzoraat is judged by changes in its colouration, etc.