I invented both the L4 and L5 systems. Most days I wish I hadnt.
Tony, an L4 system with an additional detector is not L5. It is L4 with an additional detector.
Wizard, you are misinterpreting what we wrote in the BS. It is really quite simple- it is the fire safety specialist who is meant to determine where detectors go, based on their knowledge of fire safety which fire alarm designers are not expected to have. Having decided where they are to go, back in 1988 the categories (then known as types) were invented as a shorthand way of communicating this to the designer of the fire alarm system.
I have just drafted a Factfile for FIA, which will explain this in words of one syllable, in the possibly forlorn hope that after 23 years of system categories, people might get the idea.