title: Use of [x-ms-documentdb-partitionkey] on create
body: <p>Can somebody explain to me for what the <code>x-ms-documentdb-partitionkey</code> is used at a create document call?</p>
  
  <p>What I see from the exception I notice that it has to be the same as the one extracted from the document itself.</p>
  
  <p>For example c#:</p>
  
  <p><code>_container.CreateItemAsync&lt;Data&gt;(data, partitionKey: new PartitionKey($"some_key"));</code></p>
  
  <p>Clear up:
  I do understand how partitions keys working. I only don't get why I can put this parameter on a create command (not query!) when it is still extracted from the document.</p>