To the best of my knowledge, there is no such thing as an 'AF screw thread'.
AF actually stands for 'Across Flats', and is the measurement term for the width of certain spanners jaws. A 7/16" AF spanner for example measures 7/16" across the jaws.
AF spanners were designed to fit several older type of screw threaded bolts, such as BSW, BSF and others. A 7/16 AF spanner will fit both a 3/16 BSW and 1/4 BSF bolt.
You can of course find spanners with BSW measurements on them, however, a 3/16 BSW spanner indicates that the threaded part of the bolt is 3/16" in diameter, the actual size across the jaws of the spanner is as described above, 7/16", which mirrors the size of the hexagonal head of the bolt measured across the flats, this principle is essentially the same when dealing with BSF.
The whole subject of screw threads (particularly the older ones) is quite a minefield, but I hope this brief explanation is of some help!
Mikesk
AF simply means across flats.So the size marked on the spanner is the distance between the flats of an open end spanner or the opposing sides of the nut or bolt it fits.BSW,BSF and BA spanners and sockets are not AF as such as the size stamped on them is the size of the threaded part of the bolt or nut they fit.
There is a comparison chart for the different standards in imperial and metric on the web site Baconsdozen which explains how to identify the different threads etc.