diff --git a/base_classes/NXactivity.nxdl.xml b/base_classes/NXactivity.nxdl.xml
index f354de06d2..9b8d651285 100644
--- a/base_classes/NXactivity.nxdl.xml
+++ b/base_classes/NXactivity.nxdl.xml
@@ -25,32 +25,38 @@
     <doc>
          A planned or unplanned action that has a temporal extension and for some time depends on some entity.
          
-         This class is planned be used in the future as the super class for all other activities if inheritance 
-         in base classes is supported in NeXus.
+         This class is a super class for all other activities.
     </doc>
     <field name="start_time" type="NX_DATE_TIME">
         <doc>
-             ISO 8601 formatted time code (with local time zone offset to UTC information
-             included) when this activity started.
+            Start time of this activity. It is recommended to include local time zone information.
         </doc>
     </field>
     <field name="end_time" type="NX_DATE_TIME">
         <doc>
-             ISO 8601 formatted time code (with local time zone offset to UTC information
-             included) when this activity ended.
+             End time of this activity. It is recommended to include local time zone information.
         </doc>
+        <attribute name="estimated" type="NX_BOOLEAN">
+            <doc>
+                In some cases, the end time of an activity can only be estimated. In this case,
+                this attribute shall be True.
+            </doc>
+        </attribute>
     </field>
     <field name="description">
         <doc>
              Short description of the activity.
         </doc>
     </field>
-    <group name="notes" type="NXnote">
+    <group type="NXnote" nameType="any">
         <doc>
              This can be any data or other descriptor acquired during the activity
              (NXnote allows to add pictures, audio, movies). Alternatively, a
              reference to the location or a unique identifier or other metadata file. In the
              case these are not available, free-text description.
+
+             Any number of instances of NXnote are allowed for describing extra details of
+             this activity.
         </doc>
     </group>
 </definition>
diff --git a/base_classes/NXhistory.nxdl.xml b/base_classes/NXhistory.nxdl.xml
index 9dfa25d722..8566444e9d 100644
--- a/base_classes/NXhistory.nxdl.xml
+++ b/base_classes/NXhistory.nxdl.xml
@@ -30,45 +30,28 @@
     </doc>
     <group type="NXactivity">
         <doc>
-             Any activity that was performed on the physical entity prior or during the experiment. In
-             the future, if there is base class inheritance, this can describe any activity,
-             including processes and measurements.
+             Any activity that was performed on the physical entity prior or during the experiment.
         </doc>
     </group>
-    <!--For now, one workaround would be to have NXactivity as a application definition with a subentry.
-subentry(NXsuxbentry):
-doc: |
-Any activity that was performed on the physical entity prior or during the experiment.
-definition: ["NXactivity"]-->
-    <group type="NXphysical_process">
-        <doc>
-             Any physical process that was performed on the physical entity prior or during the
-             experiment.
-        </doc>
-    </group>
-    <group type="NXchemical_process">
-        <doc>
-             Any chemical process that was performed on the physical entity prior or during the
-             experiment.
-        </doc>
-    </group>
-    <group type="NXidentifier">
+    <attribute name="identifierNAME">
         <doc>
              An ID or reference to the location or a unique (globally
              persistent) identifier of e.g. another file which gives
              as many as possible details of the history event.
         </doc>
-    </group>
-    <!--There should be more activities here, like measurement.-->
-    <group name="notes" type="NXnote">
+    </attribute>
+    <group type="NXnote" nameType="any">
         <doc>
              A descriptor to keep track of the treatment of the physical entity before or during the
              experiment (NXnote allows to add pictures, audio, movies). Alternatively, a
              reference to the location or a unique identifier or other metadata file. In the
              case these are not available, free-text description.
              This should only be used in case that there is no rigorous description
-             using the base classes above. This field can also be used to pull in any activities
+             using the base classes above. This group can also be used to pull in any activities
              that are not well described by an existing base class definition.
+
+             Any number of instances of NXnote are allowed for describing extra details of
+             this activity.
         </doc>
     </group>
 </definition>