From what I understand they accept names that are clear in what they offer. If you can not get a 'product' out of the name, then its not a FAB name. So, for instance, 'BuyFruit.co.uk' is a FAB name, because its clearly about buying fruit, but N4A.net is not, as you cant tell what its about.
All of my 3 letter names were refused.
All of my generic dictionary names were refused (none of them were nouns)
All of my other names were accepted (106 names are parked there right now)
Hope that helps
Zach