C. F. Keil and Franz Delitzsch on Numbers 18:19

Num 18:19

“All the holy heave-offerings” are not the thank-offerings (Knobel), but, as in Num 18:8, all the holy gifts enumerated in Num 18:9-18. Jehovah gives these to the priests as an eternal claim. “An eternal covenant of salt is this before Jehovah,” for Aaron and his descendants. A “covenant of salt;” equivalent to an indissoluble covenant, or inviolable contract (see at Lev 2:13).