Added note further explaining the variable model, per feedback from dbaron.

Author: tabatkins

css-variables/Overview.html

@@ -372,6 +372,20 @@ <h2 id=using-variables><span class=secno>3. </span> Using Variables</h2>
    href="#invalid-at-computed-value-time"><i>invalid at computed-value
    time</i></a>.
 
+  <div class=note>
+   <p>I'm not sure how to precisely define where/how variables can be used.
+    The Grammar section has a perfect definition, it weren't all
+    non-normative (because it's just extending the 2.1 grammar). I don't know
+    if 2.1 actually has a term for what is represented by the "term"
+    production in the 2.1 grammar.</p>
+
+   <p>Basically, though, variables are lexically substituted into the
+    property value, with restrictions on where they can appear as implied by
+    the Grammar section. You can't, for example, do something ridiculous like
+    &lsquo;<code class=css>@var $unit px; div { width: 20$unit;
+    }</code>&rsquo;.</p>
+  </div>
+
   <div class=example>
    <p>For example, the following usage is fine from a syntax standpoint, but
     results in nonsense when the variable is substituted in:</p>

css-variables/Overview.src.html

@@ -141,7 +141,13 @@ <h2 id='using-variables'>
 
 	<p>A variable is substituted for its value in the property value at computed-value time.  If a declaration, once all variables are substituted in, is invalid, the declaration is <i>invalid at computed-value time</i>.</p>
 
-		<div class='example'>
+	<div class='note'>
+		<p>I'm not sure how to precisely define where/how variables can be used.  The Grammar section has a perfect definition, it weren't all non-normative (because it's just extending the 2.1 grammar).  I don't know if 2.1 actually has a term for what is represented by the "term" production in the 2.1 grammar.</p>
+
+		<p>Basically, though, variables are lexically substituted into the property value, with restrictions on where they can appear as implied by the Grammar section.  You can't, for example, do something ridiculous like ''@var $unit px; div { width: 20$unit; }''.</p>
+	</div>
+
+	<div class='example'>
 		<p>For example, the following usage is fine from a syntax standpoint, but results in nonsense when the variable is substituted in:</p>
 
 		<pre>