From the DiMarzio pickup FAQ page:
"What is F-spacing?
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.
Why are there two different spacings?
A long time ago (in the 20th century, actually) the electric guitar world was divided between Gibson and Fender designs. One of the differences between the two was string spacing. In general, Gibson chose a narrower string spacing at the bridge than Fender, and therefore the polepieces on Gibson humbuckers were closer together than the magnets on Fender pickups. When guitar shops started installing humbuckers in the bridge position of Strats, it was obvious that the strings didn’t line up with the polepieces, and if the E strings were too far outside, the sound could suffer. Our first humbuckers followed the original Gibson spacing, and we call them standard-spaced. When we released our first humbuckers with wider spacing, Floyd Rose bridges were very popular. Floyd string-spacing is the same as Fender spacing, so we naturally called the new pickups F-spaced.
How do I know which spacing to use?
F-spaced pickups measure 2.01" (51 mm) center-to-center from the first polepiece to the sixth. Standard-spaced pickups measure 1.90" (48 mm). Although some players believe that F-spaced pickups are only for the bridge position of tremolo bridge guitars, many guitars with fixed bridges (including late 1990s Gibson Les Pauls and Epiphone LPs) should have F-spaced pickups in the bridge position. Most tremolo equipped guitars that have a nut width of 1-11/16” (43mm) or more should also use an F-spaced pickup in the neck position. If you’re replacing a bridge-position pickup and you're not sure what your string-spacing is, it's usually better to get an F-spaced model. It is not necessary for the strings to pass exactly over the center of the polepieces for best performance, but it is wise to avoid a situation where the E strings are sitting completely outside of the outer polepieces.