This is a module that is distributed with Tengine which is a distribution of Nginx that is used by the e-commerce/auction site Taobao.com. This distribution contains some modules that are new on the Nginx scene.The ngx_http_footer_filter_module is one of them.
This module implements a body filter that adds a given string to the page footer.
You might say that it provides a particular case of the http sub module in the sense that it adds something to the footer. You can do the same using the http sub module but using the footer filter should be faster since there's no string matching done on the request body.