Having modified a custom MatML xml file, and restarted NX, a black hyphen is displayed in the Used column when opening the Assign Material dialog.
This can occur when two loaded MatML files share the same material name. The list of materials will show two entries for duplicate material and the second one will display a dash in the Used column.
Hovering over the dash displays a tooltip. In NX8.0 the tooltip states: "Material already loaded with the same name". NX9 states: "Material already exists with this name".
As the duplicate material has been identified either delete the material from one of the files or rename it to make it unique.
Notes and References
OS Version: 764SP1