Charles Ellicott Commentary Numbers 18:7

"And thou and thy sons with thee shall keep your priesthood for everything of the altar, and for that within the veil; and ye shall serve: I give you the priesthood as a service of gift: and the stranger that cometh nigh shall be put to death." — Numbers 18:7 (ASV)

And within the veil—that is, the veil which separated the holy place from the most holy. The word used in this place (parocheth) is used only of the second veil. . When the outer veil or hanging is meant, the word used is masak. The reference appears to be to the whole of the priestly duties performed by Aaron and his sons, from those connected with the altar of burnt offering to those performed in the most holy place.