Charles Ellicott Commentary Leviticus 13:59

"This is the law of the plague of leprosy in a garment of woollen or linen, either in the warp, or the woof, or anything of skin, to pronounce it clean, or to pronounce it unclean." — Leviticus 13:59 (ASV)

This is the law of the plague of leprosy. — that is, the above-mentioned regulations are to guide the priests in their decisions whether a garment or leather utensil is leprous or not, and in their declaration of its being clean or defiling.