Body Layout Codes
The 'Layout' codes that are used in the lists are those commonly used in the UK by the enthusiast press and societies.  These describe the seating capacity of the bus and the general layout, such as the entrance door position, number of doors, whether convertible or full open top, etc.  Occasionally, some codes appear that are not in the list and I hope are self-explanatory, but if you want to clarify a particular entry, please let me know.  The common codes are as follows:-

BODY TYPE
CO  Convertible open top, can be operated with or without roof fitted
PO  Part open top, part covered roof (usually the forward part covered)
OO Optional open top - the bus has a sliding cover which can be used by the driver to cover the open deck.
OPO Optional part open top - part of the bus has a fixed roof, while the other has a driver-operated sliding cover.
T  Upper deck open, but a transparent cover has been fitted (leaving open sides).
Hy Bus fitted with a lifting roof mechanism.
OB  Open top single deck bus
OC  Open top single deck coach

LAYOUT/SEATING CAPACITY
For double deckers, this is shown with the top deck seating capacity first, for example 45/30, indicating 45 seats on the top over 30 on the lower deck. (72) indicates a bus with a seating capacity of 72, but where the seating split between decks is not known.. For single deckers, only the total figure is shown because these are all seated on one deck.

DOOR POSITION
C   Centre
D   Dual doors, usually at the front and in the centre, for separate entry and exit
F   Front, in front of the front axle, or Forward if behind the front axle on buses with short front overhang
R   Rear
RD  Rear, with platform doors fitted
T   Triple doors, usually at the front, centre and rear. An unusual combination in open tops, but may apply
* or +  indicates an offside (driver’s side) door fitted where this would not normally apply to a bus of that type. This normally applies to British designs now operating in other countries where the rule of the road is to drive on the right

Examples:-
For a double deck bus, O28/26R would be a permanent open top with 28 seats on the upper deck and 26 on the lower deck (28 over 26) with a rear entrance, perhaps a traditional front-engined bus with rear open platform.  CO43/31F would be a convertible open top bus (i.e. with removable roof) with 43 seats over 31, with a front entrance, such as a modern rear-engined bus.