The B7L has a vertically mounted engine "shower cubicle" which allows for a flat aisle throughout. But it didn't sell very well in the UK or Australia, so Volvo chopped the arse off and welded on the rear of a B7R coach, creating a step just before the rear axle with the engine mounted below the floor, which they called the B7RLE (LE standing for Low Entry, whereas L stands for Low floor).
They're very easy to tell apart as a B7L only has a 2/3 width rear window, a B7RLE has a full width rear window.
The B7TL (standing for 7 litre, Transverse engined, Low floor) is the double-decker chassis. This has a transverse (sidewards) mounted engine but there isn't any floor or seats above it because of the restrictive ceiling height in the lower saloon.
And so ends the tale of Volvo's B7 series (at least in the UK). EDIT: I forgot about the B7LA (where the A stands for articulated)