With the following settings:
afmscmpref.basemetricunits = 0 (i.e. millimeters)
afmscmpref.units = 2 (i.e. per locale)
SmartClient locale = any locale using the metric system

When I open an unregistered drawing in AutoCAD + SmartClient extensions, the Register Drawing popup appears with Metric - cm preselected.

I expect the default to be Metric - mm. Allowing this default to be changed based on afm_scmpref we can avoid mistakes for clients where all drawings are in mm.

Note when I switch the radiobutton to Imperial and then back to Metric, the 25.2 SmartClient UI does select mm automatically in the dropdown. This should also occur when the popup is first displayed.