This is an example of "I don't know, I don't care" in a JXL file from Trimble. (I removed most of the content to show only what's significant)

The coordinates of the measured point are in a tag called <WGS84> but the datum name is clearly defined as "ISN93", from Iceland.

#Trimble could name the tag something like <GeographicCoordinates> (in contrast to Grid coordinates that use other points). But now it is too late. I just hope they do better in the UI.