From Online Manual
description: The anchor tag produces an anchor which can then be linked. The anchor in the html is actually prefixed with "post_". This is an inline tag and no block level tags can be placed inside, however other inline tags can be if nested correctly. Please note that if creating an anchor where the id is numeric (just like [anchor=43]), the anchor will not be created.
example: [anchor=link_here]text or more BB Code[/anchor];
result: <span id="post_link_here">text or more BB Code</span>