Uffe,
Sorry about the AF term usage as that is common in inch and metric English terminology for wrench (spanner) dimensions. The AF refers to "Across Flats" so the 17mm AF is a 17mm wrench (spanner).
Something that annoys me is that most US posters quote gauge systems and temperature without noting the systems being used when posting on an international group. The system can normally be determined from context such as degrees C or F as the difference is significant but gauge sizes such as awg and swg can differ by a significant amount for some uses but are not specified just given as "g", quoting metal thicknesses in mm removes ambiguity. It doesn't help though that in these times of high metal cost mills seem to be paring the sheet metal thickness to the minimum and below I reckon to get more sheets per ton.