Re: Hang on...
"R" is for Racing.
from the Caterham website:
The ‘S’ pack is geared towards the casual road driver and includes creature comforts such as a fully-carpeted cockpit, full windscreen, hood and side screens and leather seats.
Meanwhile, the ‘R’ pack is more track-focused and is stuffed full of race-inspired goodies that will make every drive you take an event, whilst retaining its road car status. It includes a limited-slip differential, sports suspension, a four-point race harness and many other race-orientated upgrades.
A few car makers use different suffixes to denote faster-than-standard versions of their cars... the Honda Civic Type-R and Mini Cooper S come to mind.