4k HEVC: a video file contains two versions of the same video with different resolutions -> Why?

[febefe]

I noticed with several 4K files that they contain the same film twice: once in 4K (3840x2160) and once in 2K (1920x1080).

Does anyone know why this is done?

How can I find out which of the two versions is now played by the VLC-Player?

Also, only video track 1 shows me a usable picture. Video track 2 looks like a unuseable grainy test image.

Now you might think, ok, only 2K works (for whatever reason).

However, if I play a differnt file via VLC that only contains 4K, this 4K is played back without any problems.

Thanks for any help.

[Lotesdelere]

I guess for people who don't have the required hardware for playing 4k HEVC videos. Or maybe the 4k track is HDR(10) and the 2k one is not.
Hard to tell without a sample file or at least a MediaInfo report.