The letter information will determine what application the device is intended for, ie bypass, high frequency, tolerance in capacitance over a given temperature range, voltage rating, etc. I defines all the other ratings for the part, besides the value.
MuRata Erie used to give out a really nice poster with all the codes explained in a flow chart that was divided up into end use applications. Now it is spread over several web pages. Here is something similar, but in text:
Nearly every maker of capacitors has jumped into the fray and added their own types, nevermind what the Military added to the mix. Its so messy a system that without knowing the maker you can never really be sure of the code.
BTW "YB" is a general pupose coupling and bypass capacitor, +/- 10% tolerance over its useable temperature range.