CUDA, OpenCL, Metal, or OpenGL?

As of Summer 2021, Premiere Pro will no longer support OpenCL. The GPU architecture of Premiere Pro is entirely CUDA/Metal, and this is what is exposed through the GPU extensions to the effect/transition APIs. A GPU-accelerated effect may use any combination of CUDA and Metal.

Premiere Pro plug-ins have the ability to transfer frames from CUDA to OpenGL (though not always efficiently). Read more about that here.