Leviticus 6:18

Speak with Aharon and with his sons, saying: This is the law of the sin offering which is to be killed in the place where the burnt offering is killed; it shall be slain as a sin offering before the Lord; it is most sacred.

holy, among the holy sacrifices i.e., it is a member of the category of holy sacrifices [cf. comment on Genesis 9:25].

THIS IS THE LAW OF THE SIN-OFFERING. In Seder Vayikra He commanded concerning [the offerings in the following order]: the burnt-offering, the meal-offering, the peace-offerings, the sin-offering and the guilt-offering, [the reason for this order being] that at first He spoke about the freewill offerings [namely the burnt-offering, the meal-offering and the peace-offerings], and afterwards He mentioned those offerings which are obligatory upon the sinner. Here, however, [in Seder Tzav] the explanation is given first in connection with the burnt-offering and meal-offering, and only then about the sin-offering and guilt-offering, because He wished to explain the laws of the most holy offerings [in one group], since there is one law for them all, as He said [above in Verse 10] with reference to the meal-offering, it is most holy, as the sin-offering and the guilt-offering, and only afterwards He explains the law of those offerings which are less holy [namely, the peace-offerings].
Now in this section [dealing with the sin-offering] He added many new laws. [The verse] This is the law of the sin-offering teaches that there is one law for all sin-offerings, or torath (“law” or “law of”) at the beginning of a section, denotes inclusion, while hi denotes exclusion. So also the word zoth (this), as it says here in this case, zoth torath … (this is the law of …), means exclusion. meaning that even the blood of “the inner sin-offerings” sin-offerings,” that if any of their blood was sprinkled upon a garment, the garment must be washed in a holy place [i.e., in the Court of the Sanctuary], But it does not say this law with regard to “the inner sin-offerings.” Since the section, however, begins with the expression [zoth] ‘torath’ hachatath (this is ‘the law’ of the sin-offering), and the word torath denotes inclusion, we know that the same law applies to both kinds of sin-offering. [if sprinkled upon a garment] must be washed off [in a holy place]. I might think that this applies also to a sin-offering brought from a bird, Scripture therefore says zoth [‘zoth’ torath hachatath (‘this’ is the law of the sin-offering) — and the word zoth denotes exclusion]. He also states here [in the verse before us], in the place where the burnt-offering is slaughtered shall the sin-offering be slaughtered before the Eternal, thus indicating …

now the “Torah” of the sin offering, חטאת, is mentioned. If the sin offering is meant to atone for a serious sin, its blood has to offered in side the sanctuary, and none of the offering is allocated to the priests to eat. The parts normally eaten by the priests will also be burned.