/td><td>Yes</td></tr></tbody></table></section><h2 id=Subject>Subject</h2><section><p>Subject defines an identity. The identity is either a user or identified by a set of <code>properties</code>.
The supported keys in <code>properties</code> are listed in &ldquo;constraint and properties&rdquo; page.</p><table class=message-fields><thead><tr><th>Field</th><th>Type</th><th>Description</th><th>Required</th></tr></thead><tbody><tr id=Subject-user><td><code>user</code></td><td><code>string</code></td><td><p>Opt