diff --git a/docs/source/usersguide/input.rst b/docs/source/usersguide/input.rst
index 0b132275bb..0ac71716bf 100644
--- a/docs/source/usersguide/input.rst
+++ b/docs/source/usersguide/input.rst
@@ -1110,13 +1110,14 @@ Each ``material`` element can have the following attributes or sub-elements:
 
     *Default*: ""
 
-  :density:
-    An element with attributes/sub-elements called ``value`` and ``units``. The
-    ``value`` attribute is the numeric value of the density while the ``units``
-    can be "g/cm3", "kg/m3", "atom/b-cm", "atom/cm3", or "sum". The "sum" unit
-    indicates that the density should be calculated as the sum of the atom
-    fractions for each nuclide in the material. This should not be used in
-    conjunction with weight percents.
+  :density: An element with attributes/sub-elements called ``value`` and
+    ``units``. The ``value`` attribute is the numeric value of the density while
+    the ``units`` can be "g/cm3", "kg/m3", "atom/b-cm", "atom/cm3", or
+    "sum". The "sum" unit indicates that values appearing in ``ao`` attributes
+    for ``<nuclide>`` and ``<element>`` sub-elements are to be interpreted as
+    nuclide/element densities in atom/b-cm, and the total density of the
+    material is taken as the sum of all nuclides/elements. The "sum" option
+    cannot be used in conjunction with weight percents.
 
     *Default*: None