Rashi on Numbers 6:20/רש"י על במדבר ו׳:כ׳

1קדש הוא לכהן IT IS HOLY FOR THE PRIEST — The cake, the wafer and the shoulder are a heave-offering for the priest.

2על חזה התנופה means: BESIDES THE BREAST AND THE SHOULDER which are due to him (the priest) from all feast-offerings, there is to be added to the portions assigned to the priest in the case of the Nazarite’s feast-offerings, this shoulder. Because the feast-offerings of the Nazarite are included in the general rule governing feast-offerings but were excepted to become subject to a new matter — the setting apart of the shoulder for the priest — it became necessary to restore them to the general rule of שלמים by expressly stating that they are subjected also to the setting apart of the breast and thigh as are שלמים in general (Sifrei Bamidbar 37).