Add explanation on native-model-names (#255)

doc/source/user_guide/region.rst

@@ -10,11 +10,34 @@ structure is required:
 
    - Hierarchy 1:
      - region 1:
-         some attribute: some value
+         description: Some short explanation
+         iso3_codes: [ABC, DEF, ...]
      - region 2
+         description: Some short explanation
+         iso3_codes: GHI
    - Hierachy 2:
      - ...
 
-Useful examples of region attributes are ISO2/3-codes
-(https://en.wikipedia.org/wiki/List_of_ISO_3166_country_codes)
-or the list of countries included in a macro-region (i.e., a continent).
+Regions can have attributes, for example a description or ISO3-codes. If an attribute
+`iso3_codes` is provided, the item(s) are validated against a list of valid codes taken
+from the `pycountry <https://github.com/flyingcircusio/pycountry>`_ package.
+
+Naming conventions for native model regions
+-------------------------------------------
+
+Models with a coarse spatial resolution should add a model-specific identifier to the
+native model regions (e.g., `MESSAGEix-GLOBIOM 1.1|North America`) to avoid confusion
+when comparing results to other models with similar-but-different regions.
+
+.. code:: yaml
+
+   - Model A:
+     - Model A|Region 1:
+         description: Some short explanation
+
+The renaming from (short) region-names as reported by a modelling framework to such
+more verbose region names for use in model-comparison projects can be implemented
+by the :ref:`model_mapping`.
+
+If a model has a country-level resolution where disambiguation is not a concern,
+we recommend to *not add* a model identifier.