Rashi on Exodus
לחק עולם מאת בני ישראל AS AN ASSIGNED PORTION FROM THE CHILDREN OF ISRAEL — because peace-offerings belong to the “owner”, and the breast and the shoulder shall they give to the priest.
Rashi on Exodus
כי תרומה הוא FOR IT IS A HEAVE OFFERING — i.e. the breast and the shoulder form a heave-offering.