Categorize a new element :

<CREATE>
  <MEDIA>
    <INFO>...</INFO>
    <DESCRIPTIONS>...</DESCRIPTIONS>
    <CATEGORIES>
      <CATEGORY ID="12"/>
    </CATEGORIES>
  </MEDIA>
</CREATE>

If you prefer to use the path or the uniquename of the category, you can do it, the following design the same category :

<CATEGORY ID="12"/>

<CATEGORY path="/media/Products"/>

<CATEGORY name="Products"/>

Categorize an existing element :

<UPDATE>
  <MEDIA ID="25">
    <CATEGORIES>
      <CATEGORY ID="12"/>
    </CATEGORIES>
  </MEDIA>
</UPDATE>

Notice that this category will replace any previous categories assigned to the element. If you want to assign a category without losing the former categories, you must use the "append" operation.

 <UPDATE>
  <MEDIA ID="25">
    <CATEGORIES operation="append">
      <CATEGORY ID="12"/>
    </CATEGORIES>
  </MEDIA>
</UPDATE>