diff --git a/docs/source/usersguide/input.rst b/docs/source/usersguide/input.rst
index 0b132275bb..fa43ca1d2c 100644
--- a/docs/source/usersguide/input.rst
+++ b/docs/source/usersguide/input.rst
@@ -1114,8 +1114,10 @@ Each ``material`` element can have the following attributes or sub-elements:
     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
+    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