I dunno; its a reasonable attempt by someone to interpret the non plain English & ambiguities of book, written by some quasi scholars & academics, including the likes of NICEIC etc. As they all in cohorts with compiling BS7671 etc etc, and the On-site-guide or the Site Guide, shame they just can't have a directness of speech & writing in the first place.
Has got some pretty pictures, which the 'Regs' lacks.