Charles Ellicott Commentary Leviticus 7:23

"Speak unto the children of Israel, saying, Ye shall eat no fat, of ox, or sheep, or goat." — Leviticus 7:23 (ASV)

You shall eat no manner of fat. —That is, the fat of cattle, sheep, or goats. The fat of these three kinds of sacrificial quadrupeds is prohibited, even when they are not killed as sacrifices, but when slaughtered for private consumption; but the fat of other tame or wild clean quadrupeds, such as stags, roes, etc., was lawful.

According to the practice that prevailed during the Second Temple, there are three kinds of fat for the eating of which a man incurred the penalty of excision:

1. the fat which is upon the internal organs,
2. the fat upon the two kidneys, and
3. the fat upon the flanks

(Leviticus 9:10). The rump, the kidney, and the caul above the liver were not called fat, except in sacrifices. The fat which is covered with flesh is lawful; the fat upon the kidneys is forbidden; but that which is within the kidneys, as well as that of the heart, is lawful.