Adam Clarke on Leviticus 16:3

With a young bullock for a sin-offering - The bullock was presented as a sin-offering for himself, his family, the whole priesthood, and probably the Levites. The ram was for a burnt-offering, to signify that he and his associates were wholly consecrated, and to be wholly employed in this work of the ministry. The ceremonies with which these two sacrifices were accompanied are detailed in the following verses.