What is vBulletin NULL?
 

Hi, I really want to purchase a licence for myself, but after researching I found that there are many versions with many names, like 'vBulletin NULL' what is this and what's the difference between the NULL ones and the ones that sold here?

The NULL ones can be found on many websites and can be downloaded for FREE, but how does that works?

There are few websites that provide this piece of software for FREE and was wondering how does these guys distribute this software for FREE?

Websites like:
www.vbteam.info
www.vietvbb.vn

Please make me clear on this, thanks in advance.