The optionalstatic directory can be used to store any non-XML local content, for example images and prefabricated PDF. You may use subdirectories.
On inserting images into PDF, the static directory serves as the local root path to the XSLT processor. So you can either use an absolute path (URL) or a relative (local) path that refers to this directory.
If the email-body has a content type like text/html; charset=utf-8 then it may include tags such as <img src="cid:foo/bar.jpg">. The tag is most commonly an <img>, but can be any tag. The system then searches in the static directory for any file with that path. The file is included with the image, as a “related” multi-part part, meaning the file is available to the HTML document as an "inline image" when its rendered in the email client. Don't forget the cid: text in the src attribute!
Instead of returning content from XSLT transformation, you may also return “prefabricated” content from this directory. The syntax <response-from-static filename="path-to-file"/> returns a file from this directory.