purpose limitation section initial draft (#366)

Agreed on our call today.

* purpose limitation section initial draft

purpose specification principle
purpose limitation (no secondary use) principle

updated definition of secondary use

* don't nest anchor tags with definition inside a hyperlink

index.html

@@ -958,8 +958,8 @@
 must take extra technical and legal measures to ensure that this data can't be then used
 for other purposes, like to grow the service.</span></p>
 
-This is a special case of the more general principle that data should not be used for more
-[=purposes=] than the data's subjects understood it was being collected for.
+This is a special case of the <a href="#no-secondary-use">more general principle that data should not be used for more
+purposes than those specified when the data was collected</a>.
 
 Services sometimes use people's data in order to protect those or other people.
 A service that does this should explain what data it's
@@ -1771,6 +1771,29 @@
 
 </aside>
 
+## Purpose limitation
+
+<div class="practice" data-audiences="websites user-agents">
+  <span class="practicelab" id="purpose-specification">
+    When accessing personal data or requesting permission, [=sites=] and other [=actors=] should specify the [=purpose=]
+    for which the data will be used.
+  </span>
+</div>
+
+<div class="practice" data-audiences="websites user-agents">
+  <span class="practicelab" id="no-secondary-use">
+    [=Actors=] should not use personal data for purposes other than those specified. (Other uses are often called
+    [=secondary uses=].)
+  </span>
+</div>
+
+Features that are designed-for-purpose facilitate these principles by providing functionality that is only or primarily
+useful for a particular purpose. Designed-for-purpose features make it easier to explain the purpose to people, and may
+also limit the feasible secondary uses of data.
+
+[=Controlled de-identified data=] may be used for additional purposes in ways that are compatible with the specified
+purpose.
+
 ## Transparency {#transparency}
 
 <div class="practice" data-audiences="websites user-agents">
@@ -2323,7 +2346,7 @@
 <dd>The inference, evaluation, or prediction of an individual's attributes, interests, or
 behaviours.</dd>
 
-<dt>Secondary Use
+<dt><dfn>Secondary Use</dfn>
 
 <dd> Secondary use is the use of collected information about an individual without
     the individual’s consent for a purpose different from that for which the