NVAPI is NVIDIA's API for offering publicly accessible SDK interfaces around NVIDIA GPU topology, driver management, frame rendering control, GPU overclocking, and a variety of other interfaces not available under other industry standard APIs or for other NVIDIA-specific feature controls. Some Direct3D games make use of NVAPI for NVIDIA features and thus DXVK-NVAPI is about supporting the API with DXVK to properly handle those games and features running within Valve's Steam Play.
So far DXVK-NVAPI exposes some features that can help some NVIDIA-optimized games with better performance by up to a percent or two while also providing more detailed topology / system information through the NVIDIA API. DXVK-NVAPI though is just for interfacing with the NVIDIA driver API and doesn't support any conversions to any other graphics driver vendor interfaces or the like.
DXVK-NVAPI 0.3 updates the NVAPI header files against their state in the NVIDIA 465 driver series and also adds more supported methods for querying GPU system information.
DXVK-NVAPI 0.3 can be downloaded from GitHub . DXVK-NVAPI will likely be of increased importance with new Steam Play features coming around such as DLSS for Steam Play .