They are not lighting circuits, I guess because lights don't use power.(Although 1.5mm² is the minimum allowed on 'power' circuits (whatever they are).)
Oh - hang on - it says somewhere else that power circuits include lighting circuits.
I know - I'll look in Part 2 for a definition of "power circuit". Oh dear.
If I wanted to install a dedicated circuit, on a B6, to supply a number of 5-10W USB 5V charging outlets, I'd have to use 1.5mm², but a dedicated circuit, on a B6, to supply a number of 50-100W lighting points would be OK on 1mm².
Makes perfect sense, surely?
But what if the first thing the circuit served was a single light in the cupboard where the CU is? Would that make it a lighting circuit? Is there a "majority rule" kind of concept which would allow me to add 1 or 2 shaver sockets to a 1mm² lighting circuit without it becoming a "power circuit"?