Commentaire sur Le Lévitique 1:8

בני אהרן הכהנים THE SONS OF AARON THE PRIESTS — i e. only as priests : when they are ministering in their priestly dignity; but if an ordinary priest officiates in the eight garments of the High Priest his service is invalid (Sifra, Vayikra Dibbura d'Nedavah, Chapter 5 9; Zevachim 18a).

V’ETH HAPADER.’ Onkelos translated it: tarba (fat), and this is also the concensus of opinion of all commentators, the word (pader) having no companion in the Hebrew language. In my opinion the word is not a generic term for all kinds of fat, but signifies specifically the thin layer of fat which spreads over and divides between the inwards, and the word pader is one of those terms whose letters are interchangeable, thus: pader — pared (division), [and is so called] since it divides between the upper and the lower inwards. That is why our Rabbis have said¹⁰⁰Yoma 26a. that [when the limbs are taken up to the altar] the pader is spread over the throat of the animal at the place where the act of slaughter was performed, for this is considered regard for Him Who is on high, since that fat is fit to be spread and serve as a cover. It is also customary among the nobility of nations to spread it over a roast. If, however, the word pader is indeed a generic term for all kinds of fat, [it is my opinion that] fat is so called because it is the greasy substance which is “separated” from the flesh, and such in fact is the term used as an equivalent for fat by students of nature, as I will mention.¹⁰¹Further, 3:9.

הפדר, fat.