Fix language around subject verification.

index.html

@@ -1293,7 +1293,7 @@ <h3>Syntax</h3>
         <h3>Credential</h3>
         <ul>
           <li>Required properties MUST be present. For example, for a
-          <a>verifiable credential</a>, <code>type</code>,
+          <a>verifiable credential</a>, <code>type</code>
           and <code>proof</code> properties are required.</li>
           <li>Property values MUST match expectations described in this
           specification. For example, the document <code>type</code> property
@@ -1318,14 +1318,18 @@ <h3>Issuer</h3>
       <section>
         <h3>Subject</h3>
         <ul>
-          <li>The claim subject identifier must match expectations.
-          Likely, that means it is the id of a known and trusted
-          <a>verifiable presentation </a> for the subject of the claim. If the
-          entity that is subject of a claim has transmitted it to the
-          verifier, the subject may be able to prove ownership of key
-          identifying properties such as email address(es) and public
-          key(s).</li>
+          <li>The value associated with the <code>id</code> property for each
+          <a>claim</a> MUST identify a <a>subject</a> to the
+          <a>verifier</a>. For example, if a subject is identified and the
+          verifier has public key metadata related to the subject that is
+          used for authentication purposes, then the verifier MAY be able to
+          authenticate the subject via a signature generated by the subject
+          that is contained in the <a>verifiable presentation</a>.</li>
         </ul>
+        <p class="note">
+The <code>id</code> property is optional, and <a>verifiers</a> MAY use other
+properties in a claim to uniquely identify the <a>subject</a>.
+        </p>
       </section>
       <section>
         <h3>Signatures / Proofs</h3>