This post is for my own benefit as I want to put the CTCSS codes somewhere online that's easy to find.
CTCSS.....What is it? This is the description from Wikipedia...."In telecommunications, Continuous Tone-Coded Squelch System or CTCSS is a circuit that is used to reduce the annoyance of listening to other users on a shared two-way radio communications channel."
In more practical terms, it is a tone that you can't hear that is added to your transmission so that you can access a distant repeater. If you don't use the CTCSS tone then the repeater will ignore you. It should mean in theory that under lift conditions, you can open up one specific repeater as opposed to several at the same time.
The letters are used in the UK to ID the CTCSS tone required to access some repeaters.