In the British/European Standard for drainage systems (BS EN
12056-2:2000) they describe 3 or 4 general differences of drainage
systems:

System I
Single discharge stack system with partly filled branch discharge
pipes
Sanitary appliances are connected to partly filled branch
discharge pipes. The partly filled branch discharge pipes are
designed with a filling degree of 0.5 (50%) and are connected to a
single discharge stack.

System II
Single discharge stack system with small bore discharge branch pipes
Sanitary appliances are connected to small bore branch discharge
pipes. The small bore branch discharge pipes are designed with a
filling degree of 0.7 (70%) and are connected to a single discharge
stack.

System III
Single discharge stack system with full bore branch discharge pipes
Sanitary appliances are connected to full bore branch discharge
pipes. The full bore branch discharge pipes are designed with a
filling degree of 1.0 (100%) and each branch discharge pipe is
separately connected to a single discharge stack.

System IV Separate discharge stack system
Drainage systems type I, II and III may also be divided into a black
water stack serving WC's and urinals and a grey water stack serving
all other appliances.

. . .

OK, ignoring System IV (which I just included for its misplaced
apostrophe for eats shoots and leaves fans) what's what with I - III?
In particular I can't get my head around System I: 'partly filled
branch discharge pipes' - are these relatively large bore, laid with
no fall and arranged to have water standing in them permanently?

And am I correct in thinking System II is the general British (and
therefore best :-) type?

Stan the Span