XOOPS_TRUST_PATH is an important concept not only for D3 but also for the security of XOOPS.
"XOOPS_TRUST_PATH" is a constant defined in mainfile.php.
define('XOOPS_TRUST_PATH','/home/yourhome/xoops_trust_path');
- html (inside DocumentRoot)
---- kernel
---- class
---- include
---- modules
-------- forum (D3 module instantce. you can name it as you like)
---- templates_c (deprecated)
---- cache (deprecated)
---- uploads (avatar, smiley, ranks etc.)
- xoops_trust_path (out of DocumentRoot)
---- modules
-------- d3forum (D3 module class)
---- uploads (attachments etc.)
---- wraps (wraps module use it)
---- templates_c
---- cache
---- fullcache (FCH use it)