ויקרא, פרק ט״ז, פסוק כ״ה

At the peak of the Yom Kippur service, once the complex blood applications are complete, the focus shifts to the select portions of the sacrifices destined for the altar. These select parts, known as the fats, are gathered from the inner service's sin offerings. The primary approach among commentators is that this instruction acts as a collective directive, uniting the fats from both Aaron's bull and the people's goat. The Torah leaves the description general to show that both sacrifices are bound together by a single requirement [שפתי חכמים, רד צ הופמן]. A broader perspective, however, extends this duty to include the goat brought as the additional offering; while the priests eat its meat, its choice fats must still be consumed by the fire [אבן עזרא].

The timing of this action is carefully regulated. The priest only burns these fats after completing both his own burnt offering and the burnt offering of the people. This follows the standard rule that a burnt offering must always be processed before the fats of a sin offering [רלב״ג, העמק דבר].

Since the general rules for burning sin offerings are already established elsewhere, the specific instruction to burn them on Yom Kippur carries a vital lesson. It teaches that this duty is absolute and must be carried out under any circumstance. The fats are burned even if Yom Kippur falls on the Sabbath, and even if the priest or the entire community is in a state of ritual impurity [תורה תמימה, מלבי״ם, אדרת אליהו, צפנת פענח].

Finally, this burning is specifically directed to the outer altar, located in the open courtyard. It cannot take place on the inner altar within the sanctuary. That inner altar is strictly reserved for burning incense and sprinkling blood, and placing any sacrifices or animal fats upon it is explicitly forbidden [רש״י, ברכת אשר].