Add docstring

doc/user-guide/groupby.rst

@@ -294,6 +294,15 @@ is identical to
     ds.resample(time=TimeResampler("ME"))
 
 
+The :py:class:`groupers.UniqueGrouper` accepts an optional ``labels`` kwarg that is not present
+in :py:meth:`DataArray.groupby` or :py:meth:`Dataset.groupby`.
+Specifying ``labels`` is required when grouping by a lazy array type (e.g. dask or cubed).
+The ``labels`` are used to construct the output coordinate (say for a reduction), and aggregations
+will only be run over the specified labels.
+You may use ``labels`` to also specify the ordering of groups to be used during iteration.
+The order will be preserved in the output.
+
+
 .. _groupby.multiple:
 
 Grouping by multiple variables

xarray/groupers.py

@@ -155,7 +155,17 @@ class Resampler(Grouper):
 
 @dataclass
 class UniqueGrouper(Grouper):
-    """Grouper object for grouping by a categorical variable."""
+    """
+    Grouper object for grouping by a categorical variable.
+
+    Parameters
+    ----------
+    labels: array-like, optional
+        Group labels to aggregate on. This is required when grouping by a chunked array type
+        (e.g. dask or cubed) since it is used to construct the coordinate on the output.
+        Grouped operations will only be run on the specified group labels. Any group that is not
+        present in ``labels`` will be ignored.
+    """
 
     _group_as_index: pd.Index | None = field(default=None, repr=False)
     labels: np.ndarray | None = field(default=None)