Looking at the bolt carrier from the rear, as it would be in the receiver, there are two milled slots- one on each side of the carrier. On a semi, the one on he right isn't cut all the way through the rear of the carrier. The one on the left is cut through. This makes it so the sear doesn't "trip" when the bolt cycles. On a G3 carrier, both sides looks the same. The left side will actuate the auto sear.
I've read that guys have FMP or HK carriers in there PTR's, so I wondered if JLD was cutting them, or just using the untouched G3 carrier. The G3 carrier works fine with a semi pack. If one were to buy a register trigger pack/sear, they would just use the G3 carrier, but if they had the semi carrier, they'd need to locate a G3 carrier for the reg. trigger pack/sear to run.