why some APIs are internal in unity xr plugin? – VIVE Wave SDK

I’m using vive wave xr plugin 5.3.2 and unity2020 to develop a unity application.

In the unity C# script, I need to use native APIs to submit some custom frames, the API I use is WVR_SubmitFrame, but I find before calling it, I need to call WVR_RenderInit.

While, the API WVR_RenderInit is internal in wvr.cs file, so I cannot call it, why it is internal?

Also, could someone show me an example that how to render a custom frame(maybe from cloud and send to me) and display it on screen by unity C# script based on unity and vive wave xr plugin?