MNT nexusformat#1435 per NIAC2024

nxdl.xsd

@@ -660,9 +660,25 @@
 					<xs:annotation>
 						<xs:documentation>
 							String describing the engineering units.
-							The string should be appropriate for the value
-							and should conform to the NeXus rules for units.
+							The string should be appropriate for the value.
+
+							* If a unit is not provided by the list of NeXus 
+							  unit categories, instead of providing a category, 
+							  the field statement can include an example of the 
+							  units directly.
+
+							* The example does not constrain the scale of the 
+							  units. For example, if the unit is ``eV/mm``, the
+							  user could specify in a data file ``eV/cm``, or any 
+							  other unit that is convertible to the example given.
+
+							It is recommended that users and application developers 
+							check if their units and their unit examples adhere to 
+							the UDUNITS standard.  [#]_
+
 							Conformance is not validated at this time.
+
+							.. [#] https://www.unidata.ucar.edu/software/udunits/
 						</xs:documentation>
 					</xs:annotation>
 				</xs:attribute>

nxdlTypes.xsd

@@ -44,6 +44,7 @@
 
 				They should describe valid units consistent with
 				the manual section on NeXus units (based on UDUNITS).
+
 				Units are not validated by NeXus.
 			</xs:documentation>
 		</xs:annotation>
@@ -81,6 +82,7 @@
 			nxdl:NX_VOLUME
 			nxdl:NX_WAVELENGTH
 			nxdl:NX_WAVENUMBER
+			xs:string
 			" />
 	</xs:simpleType>