It’s done usually because of compatability. You can take a base layer HDR/HDR10 video, then extract the DV rpu, sync it, and inject it into the HDR stream. To answer your question, it is using both the HDR base layer with DV dynamic metada.

For webs, it’s usually P5, and the conversion is apparently good enough that it doesn’t make a difference for most people. For P7, you are going to lose the enhancement layer when encoding, but that’s the price you pay for saving space.

There are instances, especially with discs, when the film is indeed graded differently, but the good encoders/remuxers pay attention to this.

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