What do the letters and numbers mean in the addresses of Allen-Bradley PLCs?
In Allen-Bradley RSLogix 5 and RSLogix 500 PLCs, memory locations are split into data files.  The letter at the beginning of the data file name tells you the datatype.


O=Output
I=Input
S=Status
B=Boolean (Bit)
T=Timer
C=Counter
R=Control (used as storage for certain instructions)
N=Integer (16-bit)
F=Float (floating point real number)


(There are other data types as well, depending on your PLC.)

The first few data files are standard and are created automatically; one for each data type:  


O0
I1
S2
B3
T4
C5
R6
N7
F8


You can use these data files in your program, but you can also create more data files of any data type you wish, to organize your data as you see fit.