what is the rationale for "uio-howto.aux.xml" DocBook file?

just noticed for the first time the in-kernel Doc file uio-howto.aux.xml (sitting next to uio-howto.xml). the latter file is the one processed by "make htmldocs", so what is the purpose of that former one? i don't see where/how it's being picked up. any hints? am i missing something painfully obvious? rday -- ======================================================================== Robert P. J. Day Ottawa, Ontario, CANADA http://crashcourse.ca Twitter: http://twitter.com/rpjday LinkedIn: http://ca.linkedin.com/in/rpjday ========================================================================