An output path pattern is a FreeMarker expression which is used to specify the path to use when saving a generated asset using variable substitution when creating web content.

Guidelines
Output path patterns beginning with a leading slash (such as /${webapp}/content/${name}.xml) will produce paths rooted at the sandbox. Those not begining with a leading slash will produce paths relative to the current working directory when the create web content wizard is invoked.

Variables
nameThe name of the form instance data as entered by the user in the create web content wizard.
webappThe name of the webapp in which the form instance data is being created. Typically, if specifying an absolute output path pattern, the path will begin with the webapp folder (i.e. /${webapp}/...)
extensionThe default extension associated with the mime-type configured for the rendering engine template. This variable is only available for rendition ouput path patterns.
xmlThe xml instance data collected by the form.
nodeThe form instance data node.
dateThe current date at which the form instance data is being saved. Refer to the FreeMarker date reference for more information.

For example
${name}.xmlform_name.xml
${name}.${extension}form_name.html
/${webapp}/content/${name}.xml/ROOT/content/form_name.xml
${date?string("yyyy-MM-dd")}.xml2007-01-09.xml

For a more complete reference, please refer to the wiki.