diff --git a/act-rules-format.bs b/act-rules-format.bs
index db094019..36dc62d6 100644
--- a/act-rules-format.bs
+++ b/act-rules-format.bs
@@ -555,7 +555,7 @@ Definitions {#definitions}
 <dl>
   <dt><dfn>Accessibility requirement</dfn><dt>
   <dd>
-    <p>A requirement is a condition that has to be satisfied in order to conform to a standard, or to comply with a contract, policy or regulation. An accessibility requirement is a requirement aimed at improving access for people with disabilities to an ICT product.</p>
+    <p>An accessibility requirement is a requirement aimed at improving access for people with disabilities to an ICT product. In the context of ACT rules mapping, a requirement can be compulsory or advisory. When compulsory, it has to be satisfied in order to conform to a standard, or to comply with a contract, policy or regulation. When advisory, it is recommended, but not satisfying it does not lead to non-conformance or non-compliance.</p>
     <p>A common example of accessibility requirements are the WCAG success criteria. There are other standards, including W3C standards, that have recommendations for accessibility, such as WAI-ARIA and HTML. Accessibility requirements are also often found in company policies, regional standards or in legislation.</p>
   </dd>