in case anyone cares, here's my take on what it is:
SDR is graded on a standard scale of "dim screen" (not truly black) to 100 nits, iirc.
HDR content is graded on a variable brightness scale. most are from 0-1000 on the assumption that consumer grade monitors/TVs can't produce even 1000 nits constantly, which was true until recently. standard HDR has one basically gamma curve defining brightness of each and every frame in the whole movie or show.
this is suboptimal because different scenes may well be lighted differently and thus benefit from being graded differently. thus there are technologies to allow for frame-by-frame different grading. one of these standards is HDR 10+, for which there's little content. the other is Dolby Vision, which is a licensed tech but with a very low fee per unit iirc.
Cliffs Notes: HDR will get you black blacks if your TV is calibrated properly and can do HDR content. but to get the truly dazzling highlights and not compromise the mapping of brightness elsewhere you should be using Dolby Vision, and if you aren't your life is just that the much poorer for it
