www.makemkv.com

So obviously there's a difference, as not all BDXL drives can read UHD discs, I've just done a lot of googling and see people asking this Q but never getting a technical answer as to what is the difference.

i.e. a regular 1080p bluray video disc is just a regular Bluray in terms of bits on disc with a file system laid out on it in a particular way and perhaps encrypted with AACS in a particular way.

it would appear that a UHD Bluray is not just a regular BDXL in terms of bits on disc with a file system laid out in a particular way and perhaps encrypted with AACS2 in a particular way. So what is the disc? Is there a technical discussion of it anywhere?

BDXL refers to multi-layer Bluray.

However, there are two bit densities covered by "BDXL". Early drives only support the lower bit density only. Newer drives support the higher bit density used for UHD.

A UHD disc in a drive that doesn't support the higher bit density will be treated as "no disk inserted".

ok, that's what i was interested in.

Is the higher bit density only for UHD discs?

As far as I can tell, the high density isn't required for UHD, but not many use the lower bit density.

And drives that can do the high bit density will do it for writes, too, assuming your media is compatible.

By memory, the "low" bit density gives 25GB per layer, 100GB max (4 layers), while the "high" bit rate is around 30-33GB/layer, 128GB max. But I can't find my source to confirm my memory, so take that for what it is worth (slightly less than "nothing").