diff --git a/cura/Settings/ExtruderManager.py b/cura/Settings/ExtruderManager.py
index a38e5cbfe1..06342b68e5 100755
--- a/cura/Settings/ExtruderManager.py
+++ b/cura/Settings/ExtruderManager.py
@@ -408,20 +408,28 @@ class ExtruderManager(QObject):
     # This function tries to fix the problem with per-extruder-settable nozzle size and material diameter problems
     # in early versions (3.0 - 3.2.1).
     #
-    # We cannot add a setting definition of "material_diameter" into the extruder's definition at runtime
-    # because all other machines which uses "fdmextruder" as the extruder definition will be affected.
+    # In earlier versions, "nozzle size" and "material diameter" are only applicable to the complete machine, so all
+    # extruders share the same values. In this case, "nozzle size" and "material diameter" are saved in the
+    # GlobalStack's DefinitionChanges container.
     #
-    # The problem is that single extrusion machines have their default material diameter defined in the global
-    # definitions. Now we automatically create an extruder stack for those machines using "fdmextruder"
-    # definition, which doesn't have the specific "material_diameter" and "machine_nozzle_size" defined for
-    # each machine. This results in wrong values which can be found in the MachineSettings dialog.
+    # Later, we could have different "nozzle size" for each extruder, but "material diameter" could only be set for
+    # the entire machine. In this case, "nozzle size" should be saved in each ExtruderStack's DefinitionChanges, but
+    # "material diameter" still remains in the GlobalStack's DefinitionChanges.
     #
-    # To solve this, we put "material_diameter" back into the "fdmextruder" definition because modifying it in
-    # the extruder definition will affect all machines which uses the "fdmextruder" definition. Moreover, now
-    # we also check the value defined in the machine definition. If present, the value defined in the global
-    # stack's definition changes container will be copied. Otherwise, we will check if the default values in the
-    # machine definition and the extruder definition are the same, and if not, the default value in the machine
-    # definition will be copied to the extruder stack's definition changes.
+    # Lateer, both "nozzle size" and "material diameter" are settable per-extruder, and both settings should be saved
+    # in the ExtruderStack's DefinitionChanges.
+    #
+    # There were some bugs in upgrade so the data weren't saved correct as described above. This function tries fix
+    # this.
+    #
+    # One more thing is about material diameter and single-extrusion machines. Most single-extrusion machines don't
+    # specifically define their extruder definition, so they reuse "fdmextruder", but for those machines, they may
+    # define "material diameter = 1.75" in their machine definition, but in "fdmextruder", it's still "2.85". This
+    # causes a problem with incorrect default values.
+    #
+    # This is also fixed here in this way: If no "material diameter" is specified, it will look for the default value
+    # in both the Extruder's definition and the Global's definition. If 2 values don't match, we will use the value
+    # from the Global definition by setting it in the Extruder's DefinitionChanges container.
     #
     def _fixMaterialDiameterAndNozzleSize(self, global_stack, extruder_stack_list):
         keys_to_copy = ["material_diameter", "machine_nozzle_size"]  # these will be copied over to all extruders