The Blood Bowl rules used in all NAF tournaments is called the Competition Rules Pack, or CRP. This rulebook is the culmination of an evolution of the Blood Bowl rules in a process called the Living Rulebook (LRB). The LRB process reached LRB5, and then the Blood Bowl Rules Committee (BBRC) was disbanded with the publication of the CRP. In addition to the CRP, however, NAF Tournaments also allow the use of the New Teams (Chaos Pact, Slann and Underworld).
With lots of games played since the last version of the LRB, almost every combination of events has happened, so there are various Clarifications that the authors have made on various forums.
The Blood Bowl world is in a state of excitement at the moment, with a new release from Games Workshop planned in the next 12 months. We have been the most popular source for finding the official BB rules since they were removed from the GW website in Spring 2014, but GW have asked us to remove them in anticipation of the new rules being released, so that players who are new to the game do not get confused.
The CRP will continue to be used at NAF tournaments until further notice, but the document was removed from thenaf.net at the end of June 2016, as it is a GW document. There are lots of copies available for personal use if you do a quick search for them – make sure to get the Competition Rules Pack.