Hard canon is what is defined as what is said by the characters. The information provided at this level is vetted by the production team.
Soft canon is what is defined as what is seen in the visuals and in the reference books which are based on the shows and are written by or are assisted by people who were intimate with the production process, ex. Michael Okuda.
"I wouldn't really consider any of this 'hard canon,' so take it all with a grain of salt. Both bios were slapped together hastily and weren't approved by the exec producers." - Mike Sussman, Enterprise Producer, TrekBBS posts, 30 April 2005. (Sussman was referring to the bios seen in a 4th season episode of Enterprise.)
While I can understand things from the production team carrying weight while the show was still on the air and what was said was likely to be canonified if/when the subject came up in an episode or movie, that incarnation of Star Trek ended seven years ago. New technical stuff written after the point, like they Haynes manual, isn't going to have any effect on the future of Trek and so shouldn't have the same "soft canon" status. The only place prime-universe Trek continues is the novels, and the Haynes book isn't consistent with the novelverse's Enterprise-B history.