Scenario: You have a base file set to a coordinate zone of No Datum, No Projection. You input labels into the base file. In this example we'll use a general note label with northing and easting data. You also have a design file that has a specified coordinate zone assigned to it.
Issue: When you xref the base file into your design file you notice that all your labeled coordinate information has changed.
This happened to a very good friend of mine who realized (not too late) that all the coordinates for the building columns were incorrect in their sheets. This is happening because the software doesn't know how to transform a No Datum, No Projection coordinate zone drawing into one that has a coordinate zone.
The only solution was to detach the xref, assign the same coordinate zone to the base file and reattach the xref.
I tested the same scenario but I assigned a coordinate zone to my base file that was different from the coordinate zone assigned to my design file. The labels came through correctly.
Lesson learned: If possible, make sure that your base file has a coordinate zone assigned to it, even if it is the same as the base file, so that your labels will import properly.