TT points to the distinction between chelev of a kosher animal eligible for the altar and other fats, and the karet for eating forbidden chelev.
כל אוכל חלב מן הבהמה אשר יקריב ממנה אשה לה׳ ונכרתה הנפש האכלת מעמיה — כרת על אכילת חלב של בהמה הראויה לקרבן.
“For whoever eats fat from the animal of which an offering by fire is made to the Lord, the soul that eats it shall be cut off from its people” — karet for eating the chelev of an animal that is eligible for the altar.
Custom — scope of the chelev prohibition and its penalty.
Eating the sacrificial fat of an altar-eligible animal incurs karet. Fat of a neveilah or treifah may be used for work but may not be eaten.