@@ -14,13 +14,16 @@ enumerated list of options for defining, for example:
 - These may have alternative naming conventions supported through aliases 
   (e.g., "235U" and "U_235" for Uranium 235).
 
-Identifiers are a list of possible valid labels. Each label has up to four
-representations:
+Identifiers are a list of possible valid options. Each option has three representations
+that are stored in an IDS:
 
 1. An index (integer)
 2. A name (short string)
 3. A description (long string)
-4. List of aliases (list of short strings)
+
+.. seealso::
+    `Data Dictionary documentation for identifiers
+    <https://imas-data-dictionary.readthedocs.io/en/latest/identifiers.html>`__
 
 
 Identifiers in IMAS-Python
@@ -32,6 +35,20 @@ constructed on-demand from the loaded Data Dictionary definitions.
 All identifier enums can be accessed through ``imas.identifiers``. A list of
 the available identifiers is stored as ``imas.identifiers.identifiers``.
 
+Each identifier option provides the following attributes:
+
+- ``name``: the name of the option.
+- ``index``: the integer index value of the option.
+- ``description``: a longer string describing the option.
+- ``aliases``: a list of aliases that can be used instead of the name.
+- ``units``: optional information about the units of the quantities that are affected by
+  the identifier option. Take, for example, the `poloidal plan coordinate identifier
+  <https://imas-data-dictionary.readthedocs.io/en/stable/generated/identifier/poloidal_plane_coordinates_identifier.html>`__
+  which affects the units of ``grid/dim1`` and ``grid/dim2``.
+
+.. versionadded:: 2.1.0 ``aliases`` for identifiers.
+.. versionadded:: 2.3.0 ``units`` metadata.
+
 .. code-block:: python
     :caption: Accessing identifiers
 
@@ -47,6 +64,9 @@ the available identifiers is stored as ``imas.identifiers.identifiers``.
     print(csid.total.index)
     print(csid.total.description)
 
+    # Search identifier options by their index value
+    print(csid(1))
+
     # Access identifiers with aliases (when available)
     mid = imas.identifiers.materials_identifier
     print(mid["235U"].name)        # Access by canonical name
@@ -57,12 +77,12 @@ the available identifiers is stored as ``imas.identifiers.identifiers``.
     assert mid["235U"].name is mid.U_235.name
 
     # Item access is also possible
-    print(identifiers["edge_source_identifier"])
+    print(imas.identifiers["edge_source_identifier"])
 
     # You can use imas.util.inspect to list all options
-    imas.util.inspect(identifiers.ggd_identifier)
+    imas.util.inspect(imas.identifiers.ggd_identifier)
     # And also to get more details of a specific option
-    imas.util.inspect(identifiers.ggd_identifier.SN)
+    imas.util.inspect(imas.identifiers.ggd_identifier.SN)
 
     # When an IDS node is an identifier, you can use
     # metadata.identifier_enum to get the identifier
@@ -186,6 +206,7 @@ material_identifier["235U"].
     mat.names[0] = mid.U_235.name  # enum value via alias
     mat.names[0] = mid["U_235"].name  # enum value via alias
 
+
 Compare identifiers
 -------------------
 
 
@@ -4,7 +4,7 @@
 
 import logging
 from enum import Enum
-from typing import Iterable, List, Type
+from typing import Iterable, List, Type, Optional
 from xml.etree.ElementTree import fromstring
 
 from imas import dd_zip
@@ -15,19 +15,29 @@
 class IDSIdentifier(Enum):
     """Base class for all identifier enums."""
 
-    def __new__(cls, value: int, description: str, aliases: list = []):
+    name: str
+    """Name of this identifier value."""
+    index: int
+    """Unique index for this identifier value."""
+    description: str
+    """Description for this identifier value."""
+    aliases: list[str]
+    """Alternative names for this identifier value."""
+    units: Optional[str]
+    """Units of the quantity/quantities altered by this identifier. May be ``None`` if
+    the Data Dictionary doesn't provide this metadata."""
+
+    def __new__(
+        cls, value: int, description: str, aliases: list[str], units: Optional[str]
+    ):
         obj = object.__new__(cls)
         obj._value_ = value
+        obj.index = value
+        obj.description = description
+        obj.aliases = aliases
+        obj.units = units
         return obj
 
-    def __init__(self, value: int, description: str, aliases: list = []) -> None:
-        self.index = value
-        """Unique index for this identifier value."""
-        self.description = description
-        """Description for this identifier value."""
-        self.aliases = aliases
-        """Alternative names for this identifier value."""
-
     def __eq__(self, other):
         if self is other:
             return True
@@ -68,19 +78,22 @@ def __eq__(self, other):
     def _from_xml(cls, identifier_name, xml) -> Type["IDSIdentifier"]:
         element = fromstring(xml)
         enum_values = {}
-        aliases = {}
         for int_element in element.iterfind("int"):
             name = int_element.get("name")
             value = int_element.text
+            assert value is not None
             description = int_element.get("description")
             # alias attribute may contain multiple comma-separated aliases
             alias_attr = int_element.get("alias", "")
             aliases = [a.strip() for a in alias_attr.split(",") if a.strip()]
-            # Canonical entry: use the canonical 'name' as key
-            enum_values[name] = (int(value), description, aliases)
-            # Also add alias names as enum *aliases* (they become enum attributes)
+            units = int_element.get("units")
+
+            # Identifier can be looked up by its name or any of its aliases:
+            enumvalue = (int(value), description, aliases, units)
+            enum_values[name] = enumvalue
             for alias in aliases:
-                enum_values[alias] = (int(value), description, aliases)
+                enum_values[alias] = enumvalue
+
         # Create the enumeration
         enum = cls(
             identifier_name,
 
@@ -6,11 +6,9 @@
 from imas.ids_factory import IDSFactory
 from imas.ids_identifiers import IDSIdentifier, identifiers
 
-has_aliases = Version(importlib.metadata.version("imas_data_dictionaries")) >= Version(
-    "4.1.0"
-)
-requires_aliases = pytest.mark.skipif(
-    not has_aliases, reason="Requires DD 4.1.0 for identifier aliases"
+requires_dd4_1 = pytest.mark.skipif(
+    Version(importlib.metadata.version("imas_data_dictionaries")) < Version("4.1.0"),
+    reason="Test requires DD 4.1.0 for additional identifier metadata",
 )
 
 
@@ -112,7 +110,7 @@ def test_identifiers_with_aliases():
     assert identifier.CxHy.aliases == ["alias1", "alias2", "3alias"]
 
 
-@requires_aliases
+@requires_dd4_1
 def test_identifier_struct_assignment_with_aliases():
     """Test identifier struct assignment with aliases using materials_identifier."""
     mid = identifiers.materials_identifier
@@ -174,7 +172,7 @@ def test_invalid_identifier_assignment():
         cs.source[0].identifier = -1
 
 
-@requires_aliases
+@requires_dd4_1
 def test_identifier_aliases():
     """Test identifier enum aliases functionality."""
     mid = identifiers.materials_identifier
@@ -197,7 +195,7 @@ def test_identifier_aliases():
         assert mid[alias] is mid.U_235
 
 
-@requires_aliases
+@requires_dd4_1
 def test_identifier_alias_equality():
     """Test that identifiers with aliases are equal when comparing names and aliases."""
     mid = identifiers.materials_identifier
@@ -276,7 +274,7 @@ def test_identifier_alias_equality():
     assert mat5.descriptions[2] == mid["U_235"].description
 
 
-@requires_aliases
+@requires_dd4_1
 def test_identifier_alias_equality_non_ggd():
     """Test identifier aliases functionality on non-ggd material"""
     mid = identifiers.materials_identifier
@@ -293,3 +291,14 @@ def test_identifier_alias_equality_non_ggd():
     summary_ids.wall.material.name = "235U"  # Use canonical name
     assert summary_ids.wall.material == mid["235U"]
     assert summary_ids.wall.material == mid["U_235"]
+
+
+@requires_dd4_1
+def test_identifier_units():
+    ppcid = identifiers.poloidal_plane_coordinates_identifier
+    assert ppcid.rectangular.units == "m,m"
+    assert ppcid.inverse.units == "m,rad"
+
+    # materials identifier doesn't have units (and I don't expect they'll ever get any)
+    mid = identifiers.materials_identifier
+    assert mid.W.units is None