/htdocs is the directory accessible directly via http request. In other words - it is the directory pointed by DocumentRoot apache directive in your VirtualHost definition.

/htdocs is the place for:

You can get the full absolute path to /htdocs_dir by

$WSC->get_htdocs_dir();

in global scope or

$this->get__config()->get_htdocs_dir();

in page scope.