For me theres two different topics here.
The first is Part 1 vs Part 6 - which i think is pretty black and white. You are given a 'menu' of design categories (and in the latter case, a grade also) to choose from menu style, and i cant see why it has to be anymore complex than this? Pick one, and work to one or the other depending on if the property is a dwelling or not.
This slightly leads on to topic two - Potentially if its a part 1 system, i could understand this becoming a little more obscure if an L5 system is specified - however, the selection of L1 to L4 (from the 'menu') should be decdied upon a Risk Assessment just as much as an L5 in which case some specific guidlines as to what the objective of the L5 system should be given. (Whats to be protected and how to do it, or if the system is an engineering solution).
Whomever needs to design/install the system, can then do-so accordingly.
Before anyone says, this is all very idealistic i know and appreciate...
I do throw out a question to everyone though - Is it just me or do very few risk assessments actually conclude a design category? Very few and far between from what i have seen.