A detent pin anvil has a retaining pin that must be pressed in to release a socket from the impact driver. A hog ring anvil has a ring that exerts pressure on the socket and can be removed without tools (or with bare hands).
The difference is in application- typically a user in an elevated position who is driving the same sized nuts or bolts, will use the detent pin anvil. This is common for applications such as bridge or building assembly, water tower assembly and other construction type projects. In these applications, the detent pin will prevent the socket from falling out unexpectedly.
The hog ring anvil is used by those who need to quickly change, such as a mechanic. The mechanic working on a vehicle will need to quickly change out the sockets without the extra step of using another tool to release the socket from the anvil. In this application, it's usually low to the ground, so a socket that falls out will not pose a safety hazard to those below.
DEWALT impact drivers with the hog ring will contain the letter H, for example: DCF889HM2 or DW059HK-2. In some situations, the anvil may be converted. Please consult a factory service center.
Detent pin anvil:
Hog ring anvil: