The SLC and PLC5 have two methods of looking at binary file addresses.
B3/16 is the same as B3:1/0 its simple the 16th bit in the file starting at B3:0/0. B3:0/15=B3/15 etc. You can switch it if you right click in the ladder and go to Address Display options. BTW IO can do the same thing.
The only programming advantage is if you want to indirectly address more then 16 bits in a B file. For example B3/[N7:0] if N7:0=16 would point to B3:1/0.