The melocha of borer, separating the bad from the good, applies not only to something which is objectively bad such as a pit or a rock, but even to something subjectively bad. For example, if one does not like carrots in his salad, to remove them will transgress the melocha. (To remove them to give to someone else that wants them is allowed).
Borer applies only to a mixture. Even if the mixture is of big pieces where the different items are easily discernable, it is still forbidden to remove the unwanted pieces. For example, a plate of chicken legs and thighs unevenly dispersed, it is forbidden to remove the legs if your intentions are to eat the thighs. (see Rama 319:3) If, however, the items are separated into neat obvious groups, borer doesn’t apply. For example, a deli platter with the different types of meat lined up each meat in its own group, any kind of meat may be separated from the rest of the plate.
Borer is only with two different types of objects. Two of the same objects however is exempt from the melocha. Therefore, a plate of many chicken legs, even big and small pieces, is not applicable to the melocha.