diff --git a/doc/source/operations.rst b/doc/source/operations.rst
index 0bbf599..a157403 100644
--- a/doc/source/operations.rst
+++ b/doc/source/operations.rst
@@ -92,6 +92,11 @@
 can be assigned to each mapping by providing a space-separated list (class
 names cannot contain spaces anyway).
 
+Note that mappings are not designed to replace node definitions. Mappings can
+be used to pre-populate the classes of existing nodes, but you still need to
+define all nodes (and if only to allow them to be enumerated for the
+inventory).
+
 Parameter interpolation
 ------------------------
 Parameters may reference each other, including deep references, e.g.::