What are the utilization "domains" for NvAPI_GPU_GetDynamicPstatesInfoEx?

The NvAPI documentation states:

There are currently 4 domains for which GPU utilization and dynamic P-State thresholds
can be retrieved:  graphic engine (GPU), frame buffer (FB), video engine (VID), and bus
interface (BUS).

Each domain's info is indexed in the array.  For example:
holds the info for the GPU domain.

But there are no definitions for NVAPI_GPU_UTILIZATION_DOMAIN_GPU or any other manifest constants, so it is not obvious which values are being returned by NvAPI_GPU_GetDynamicPstatesInfoEx.

Can somebody please document this?