Allow for open enumerations

base_classes/NXobject.nxdl.xml

@@ -51,7 +51,7 @@
 				For example, all IGSNs (see below) are DOIs and all DOIs are Handles; however, an IGSN should have type IGSN (and not DOI or Hdl).
 				Similarly, an ARK, Purl, ORCID and ROR identifiers should have their corresponding types and should not use the more generic URL identifier.
 			</doc>
-			<enumeration>
+			<enumeration open="true">
 				<item value="ARK">
 					<doc>
 						Archival Resource Key.

base_classes/NXsensor.nxdl.xml

@@ -50,7 +50,7 @@
 	</group>
 	<field name="measurement">
 		<doc>name for measured signal</doc>
-		<enumeration>
+		<enumeration open="true">
 			<item value="temperature" />
 			<item value="pH" />
 			<item value="magnetic_field" />

base_classes/NXsource.nxdl.xml

@@ -47,7 +47,7 @@
 	</field>
 	<field name="type">
 		<doc>type of radiation source (pick one from the enumerated list and spell exactly)</doc>
-		<enumeration>
+		<enumeration open="true">
 			<item value="Spallation Neutron Source" />
 			<item value="Pulsed Reactor Neutron Source" />
 			<item value="Reactor Neutron Source" />
@@ -165,10 +165,9 @@
 	</group>
 	<field name="mode">
 		<doc>source operating mode</doc>
-		<enumeration>
+		<enumeration open="true">
 			<item value="Single Bunch"><doc>for storage rings</doc></item>
 			<item value="Multi Bunch"><doc>for storage rings</doc></item>
-			<!-- other sources could add to this -->
 		</enumeration>
 	</field>
 	<field name="top_up" type="NX_BOOLEAN">

dev_tools/docs/nxdl.py

@@ -512,10 +512,16 @@ def _print_enumeration(self, indent, ns, parent):
         if len(node_list) == 0:
             return ""
 
-        if len(node_list) == 1:
-            self._print(f"{indent}Obligatory value:", end="")
+        if parent.attrib.get("open", "false") == "true":
+            self._print(
+                f"{indent}Any of these values or a custom value (if you use a custom value, also set @custom=True):",
+                end="",
+            )
         else:
-            self._print(f"{indent}Any of these values:", end="")
+            if len(node_list) == 1:
+                self._print(f"{indent}Obligatory value:", end="")
+            else:
+                self._print(f"{indent}Any of these values:", end="")
 
         docs = OrderedDict()
         for item in node_list:

nxdl.xsd

@@ -1232,7 +1232,7 @@ https://stackoverflow.com/a/48980995/1046449 -->
 
 				(This data type is used internally in the NXDL schema 
 				to define elements and attributes to be used by users in NXDL specifications.)
-				
+
 				::
 
 					<field name="mode">
@@ -1244,6 +1244,20 @@ https://stackoverflow.com/a/48980995/1046449 -->
 						</enumeration>
 					</field>
 
+				Enumerations can be closed or open. A closed enumeration is one where the list of
+				values are the only ones allowed for a specification. An open enumeration is one 
+				where the list of values is incomplete and additional values other than those listed
+				are allowed. Open enumerations should be used sparingly as the designer of the enumeration
+				should try to find all possible values for a given field/attribute.
+
+				In case an open enumeration is used and the data provider wants to declare that the provided
+				value deliberately not from the given list of values, the data provider should,
+
+				* for fields, set the attribute ``@custom=True``.
+				* for attributes, set an additional attribute ``@my_attribute_custom=True``, where
+				  ``my_attribute`` is the name of the attribute with the open enumeration.
+
+				Such attributes can be used deliberately to suppress warnings in NeXus validators.
 			</xs:documentation>
 		</xs:annotation>
 		<xs:sequence>
@@ -1279,6 +1293,14 @@ https://stackoverflow.com/a/48980995/1046449 -->
 				</xs:complexType>
 			</xs:element>
 		</xs:sequence>
+		<xs:attribute name="open" use="optional" type="nx:NX_BOOLEAN" default="false" >
+			<xs:annotation>
+				<xs:documentation>
+					Is this an open enumeration?
+					An open enumeration allows additional values not listed.
+				</xs:documentation>
+			</xs:annotation>
+		</xs:attribute>
 	</xs:complexType>
 
 	<xs:complexType name="dimensionsType">