Updated function definition

Updated function definition according to issue #181

model/index.html

@@ -377,10 +377,10 @@ <h3>Party</h3>
 
 <section id="function">
 <h4>Function</h4>
-
- <p>The abstract <code><em>function</em></code> property is used to link a Rule to a Party, indicating the function undertaken by the Party in respect to the Rule that links to it. The <code><em>function</em></code> property is represented by explicit subtypes that indicate the semantics of the functional role between the Party and the Rule.</p>
 
-<p>The <code><em>function</em></code> property MUST support the following subtypes:</p>
+<p>A <code><em>function</em></code> property is used to link a Rule to a Party, indicating the function undertaken by the Party in respect to the Rule that links to it. The <code><em>function</em></code> property itself is abstract; subproperties represent explicit semantics of the functional role between the Party and the Rule.</p>
+
+<p>The most important sub properties of <code><em>function</em></code> are:</p>
 <ul>
 <li><code>assigner</code>: indicates the Party that is issuing the Policy. For example, the Party granting a Permission or requiring an agreed Duty to be performed.</li>
 <li><code>assignee</code>: indicates that the Party that is the recipient the Policy. For example, the Party being granted a Permission or required to perform an agreed Duty.</li>