Navigation system

Can anyone tell me what the letter preceding the place name indicates on the 2002 E46 navigation system? When I have the split screen option in use there
is a readout of the current position. If I am travelling through the town of Stirton for example it might say S-Stirton. Sometimes I also notice the prefix L or U used also. I can't find anything in the handbook for the system which explains what this means.
Thanks David

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