Why/when is XMP::xmpMM\InstanceID being created/filled?

[ubacher]

I notice that files get marked for pending rewrite (yellow pencil) with the reason being
XMP::xmpMM\InstanceID

Is this something new Imatch generates? And when?

[jch2103]

I've noticed this also, along with some other tags. I've ignored it, but assume it's from some recent ExifTool update?

[Mario]

This id must be added/updated when XMP data changes. Totally normal.

[ubacher]

This id must be added/updated when XMP data changes. Totally normal.

But new in 2021?
(I did read release notes - no mention though.)