The distance between the strings on a regular bridge is different to that of a floyd rose bridge.

The F spaced pickups are designed so that the little pole pieces are directly under the strings on a Floyd rose equipped guitar.
It actually comes from "Fender Spaced" as Fender's string spacing at the bridge was greater than Gibson. So when people started putting Humbuckers in Strats, they noticed the pole pieces didn't line up. So they made "F-Spaced" humbuckers to put on their strats.

Floyd Rose bridges have the same string spacing as Fender bridges, so it is either or, but it started as Fender, not Floyd.
If your guitar has a tremolo, or a Fender spaced hard-tail, you get F-Spaced.

All of our full-size humbuckers except the X2N® are available in two polepiece spacings. F-spacing refers to the wider of the two spacings. For proper string alignment and balanced output, F-spaced humbuckers should be used in the bridge position on all guitars with string spacing at the bridge of 2.1" (53 mm) or greater. On these guitars, if the nut width is 1-11/16? (43 mm) or greater, F-spaced pickups can be used in the neck position as well.

From DiMarzio
As far as the string spacing over the pickups is concerned, 48mm is standard spaced, 51.5mm is F-spaced.
Actually called Mark!

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