what's the use of define('INCLUDED',1)

i just want to know how this line of code is being used by the forum and what is its importance

It is a very important line, and it should be included in any kind of module. It means the module it is contained in, can not be executed externally, but only through index.php where this variable is defined. For example if you type

http://www.minibb.com/forums/bb_func_vtopic.php

it will be not possible to execute it.

that's genius.... makes a lot of sense..

thanks.. miniBB rocks