Here is the other end of the duct where it terminates near the CV joint.
The 992 GT3 rear diffuser is much larger than the 991 part and has a more aggressive up turn at its end. This is probably where a lot of the downforce improvements come from. The 991 diffuser is minimalistic compared to this.
There are 3 vents in the diffuser that feed air to the exhaust particulate trap/cat area and the PDK transmission. Interestingly the 991.2 GT3 didn’t have any venting in the diffuser for the PDK at all. We sort of feel that these vents are sort of in a bad place when it comes to diffuser effectiveness but good for cooling.
The two side vents feed air over the particle traps and the cats. The GT3 actually has a particle trap just like a diesel. Apparently, Porsche found that direct-injected engines make particulates like a diesel. We assume that the car probably has some sort of regeneration cycle to clean the traps that makes a lot of heat, hence this metal-lined scoop.