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Did the sketch create an intermediate datums CSYS (that then got hidden)?
Right-click on the sketch *feature* -> is there a menu item "Make Datums External"? - if so, select that.
Then examine the datum CSYS that shows up
Production: NX10.0.3.5 MP16/TC11.2
I'd rather be e-steamed than e-diseaseled
I would expect the offset parameter to belong to the datum plane and not the sketch itself. The planar object that you sketch on is technically not part of the sketch. The sketch dialog helpfully allows you to specify or create a plane for the sketch, but the plane object must exist before the sketch in the feature tree; the plane will be a parent of the sketch. Making the datum "internal" to the sketch hides this relationship (not a good thing, IMHO).
I know that a sketch can be "internal" or "external" to an Extrude or Revolve feature. To speak frankly, this is the first time I know that a datum can be "internal" or "external" to a sketch. In my understanding what you mean by "internal" or "external" is about the behind-the-scence mechanism, but not about real operation? And I never see a menu item "Make Datums External".
"And I never see a menu item "Make Datums External"."
Sorry, I'm still on NX 9 at the moment and my answer referenced functionality in that version. There were changes in either NX 10 or 11 that may make that particular part of my answer obsolete.