sharepoint rest api – What’s the difference between the fields “owshiddenversion” and “OData__UIVersion”?

Query is something like .../_api/web/Lists/GetByTitle('Documents')/items?$select=File,File/ServerRelativeUrl,OData__UIVersion,owshiddenversion,FileRef

This worked in a SP Root site: OData__UIVersion was the exact number of versions.

But in a subsite (the same SP), for a file with just one version, OData__UIVersion is 512

Next, I discovered the other field: owshiddenversion; This one seems to really return the correct value: the number of versions of a file.

Does anybody know what’s the official definition of these 2 fields ?
Is it safe to rely on owshiddenversion to determine how many versions a file has ?

Thank you.