In ./doku.php the global $INFO is set up with:
//make infos about the selected page available
$INFO = pageinfo();
That makes it easy to do various things all over the place.
However, it is not set up in all the other php entry points, leading to confusion of when it can be used, and when it can't be. For example - its missing in ./lib/exe/detail.php, which means in the template's detail.php, you can't print out the username of the person logged in at the top of the page. (if you want ./lib/tpl/xxx/main.php and ./lib/tpl/xxx/detail.php to have similar header infos).
It looks like someone was going to do this, but didn't finish (as the code is commented out)...