extends attribute to clarify if a field in a specified NXdata is a AXISNAME or DATA

contributed_definitions/NXiv_temp.nxdl.xml

@@ -71,9 +71,9 @@
                  There should also be a field with an array of rank equal to the number of different temperature setpoints and each child's dimension
                  equal to the number of voltage setpoints.
             </doc>
-            <field name="temperature" type="NX_NUMBER"/>
-            <field name="voltage" type="NX_NUMBER"/>
-            <field name="current" type="NX_NUMBER">
+            <field name="temperature" type="NX_NUMBER" extends="/NXdata/AXISNAME"/>
+            <field name="voltage" type="NX_NUMBER" extends="/NXdata/AXISNAME"/>
+            <field name="current" type="NX_NUMBER" extends="/NXdata/DATA">
                 <dimensions rank="2">
                     <dim index="1" value="n_different_temperatures"/>
                     <dim index="2" value="n_different_voltages"/>

dev_tools/docs/nxdl.py

@@ -607,6 +607,12 @@ def _print_attribute(self, ns, kind, node, optional, indent, parent_path):
         )
         self._print_doc_enum(indent, ns, node)
 
+    def _get_extends_text(self, node):
+        extends = node.get("extends", None)
+        if extends:
+            return f"(:ref:`{extends.split("/")[-1]} <{extends}-field>`) "
+        return ""
+
     def _print_if_deprecated(self, ns, node, indent):
         deprecated = node.get("deprecated", None)
         if deprecated is not None:
@@ -630,11 +636,13 @@ def _print_full_tree(self, ns, parent, name, indent, parent_path):
             dims = self._analyze_dimensions(ns, node)
 
             optional_text = self._get_required_or_optional_text(node)
+            extends_test = self._get_extends_text(node)
             self._print(f"{indent}{self._hyperlink_target(parent_path, name, 'field')}")
             self._print(f"{indent}.. index:: {index_name} (field)\n")
             self._print(
                 f"{indent}{formatted_name}: "
                 f"{optional_text}"
+                f"{extends_test}"
                 f"{self._format_type(node)}"
                 f"{dims}"
                 f"{self._format_units(node)}"

nxdl.xsd

@@ -963,6 +963,44 @@ https://stackoverflow.com/a/48980995/1046449 -->
 						</xs:restriction>
 					</xs:simpleType>
 				</xs:attribute>
+				<xs:attribute name="extends" use="optional">
+					<xs:annotation>
+						<xs:documentation>
+							The ``extends`` attribute allows to specify which field
+							in the inheritance hierarchy this field is derived from.
+
+							This attribute is to be used in cases where there is an ambiguity
+							in the inheritance. At the moment, this is only the case for the
+							``AXISNAME`` and ``DATA`` fields in ``NXdata``, which are both of
+							type ``NX_NUMBER``. Thus, when specializing ``NXdata`` in an application
+							definition, the ``extends`` attribute allows to specify whether a field
+							in the specialized ``NXdata`` is an ``AXISNAME`` and ``DATA``.
+
+							For example, consider the following NXDL snippet::
+
+								&lt;definition xmlns=&quot;http://definition.nexusformat.org/nxdl/3.1&quot; xmlns:xsi=&quot;http://www.w3.org/2001/XMLSchema-instance&quot; category=&quot;application&quot; name=&quot;NXexample&quot; extends=&quot;NXobject&quot; type=&quot;group&quot; xsi:schemaLocation=&quot;http://definition.nexusformat.org/nxdl/3.1 ../nxdl.xsd&quot;&gt;
+									&lt;doc&gt;Example application definition.&lt;/doc&gt;
+									&lt;group type=&quot;NXentry&quot;&gt;
+										&lt;group type=&quot;NXdata&quot;&gt;
+											&lt;doc&gt;
+												This NXdata contains multiple fields. ``temperature`` and ``voltage``specialize ``AXISNAME``, whereas ``current`` specializes ``DATA``.
+											&lt;/doc&gt;
+										&lt;field name=&quot;temperature&quot; type=&quot;NX_NUMBER&quot; extends=&quot;/NXdata/AXISNAME&quot;/&gt;
+										&lt;field name=&quot;voltage&quot; type=&quot;NX_NUMBER&quot; extends=&quot;/NXdata/AXISNAME&quot;/&gt;
+										&lt;field name=&quot;current&quot; type=&quot;NX_NUMBER&quot; extends=&quot;/NXdata/DATA&quot;/&gt;																		 
+										&lt;/group&gt;
+									&lt;/group&gt;
+								&lt;/definition&gt;
+
+							Here, the ``extends`` attribute is used to specify that the ``temperature`` and ``voltage`` fields are specializations
+							of the ``AXISNAME`` field, whereas the ``current`` field is a specialization of the ``DATA`` field.
+
+							Note that the ``extends`` keyword shall only be used in cases where there is an ambiguity in the inheritance, such as
+							the example above. The ``extends`` attribute must not be used when such ambiguity does not exist. Especially, it is
+							not alllowed to specify the ``extends`` attribute for fields that are not part of the inheritance hierarchy.
+						</xs:documentation>
+					</xs:annotation>
+				</xs:attribute>
 				<xs:attributeGroup ref="nx:nameTypeAttributeGroup"/>
 			</xs:extension>
 		</xs:complexContent>