There are several types of fat you will encounter on any piece of meat. The sloppy, loose fatty connective tissue, that doesn't cook up right. Soft and hard yellow fat, this is a sign of oxidation, it will not taste good. Then there is hard, white fat. This is fat that is not oxidized, it will have the sweet flavor that fat at it's best possesses. It is also a sign that the meat has not been exposes to excess oxygen, meat that has been exposed to oxygen for more than a few hours will begin to decay, soft yellow fat means it has been exposed for many hours.
When I buy any brisket, I look first at the fat, even in cryo, I can see if it is hard, white and tends to break into leafs, or yellow, soft and pliable. First indication of a cut that will taste better. Fat is largely a molecular form of sugar, stored in a cellular structure of proteins. As it breaks down, the sugars turn rancid, yellowing and taking on off flavors.