B.S.F. likely refers to "British Standard Fine" nuts/boltheads. As I understand it, the British used differently sized nuts/boltheads for fine thread (BSF) than for standard thread (British Standard Whitworth, BSW). You'll often find "Whitworth" wrenches marked "[X size] BSW/[Y size] BSF," referring to the major diameter of the thread, NOT to the nut dimension. So the 1/2" BSF max on the range would mean it fits the nut for a 1/2" BSF bolt, although it's pretty short for removing a well-torqued/well-rusted nut that size.
Imagine being a British car/motorcycle mechanic, machinery mechanic, millwright, etc., and having to pick the right wrench for a nut you encountered on a repair. You'd probably get pretty good at it, just as mechanics/repairfolk working on equipment with inch/fractional and metric parts: look at it, pick the right wrench. But it might take longer.
