When I was a mechanic AF stood for “across flats” and actually applies to both metric and imperial spanners although the term AF was not actually used in practice very often and only for imperial spanners.
The term “AF system” is mentioned several times here:
http://classicmechanic.blogspot.com/201 ... chart.htmlBolt sizes like British Standard Whitworth confuse things but you can still usually deal with the fasteners once you know what the Across Flats size is but failing that look for a BSW spanner that looks about the right size!
A slightly further confusion is that some metric thread fasteners can have more than one across flats head size.
When I worked as a Ford Mechanic the most common spanners and sockets needed were: 10, 13, 15, 17 and 19mm.
Some manufacturers, possibly including Ford these days, now require 12, 14 and 16mm tools amongst others.
I hate to imagine what the Americans are using these days ...