Jeppesen have an explination for all the numbers, I can't remember it, but like there are numbers for VOR/DME and NDB etc.
To answer Old Smokey's question, the first number 10-9 in this case '1' is the number given to the airport in a said location. So if you look at London, there's 10-9, 20-9, 30-9, 40-9 & 50-9 all being the airport layout diagram for LHR, LGW, STN, LCY & LTN.
MK