Gibson and Fender guitars have different string spacing at the bridge. A “regular-spaced” humbucker refers to Gibson spacing of the strings at the bridge. A “F-spaced” humbucker is “Fender” spacing. The bridge spacing directly correlates to the pole spacing on the pickups. You want the pole pieces to be directly under the strings. That’s why you need to match up your bridge spacing with the pickup.
DiMarzio uses the term “F-spaced” while Seymour Duncan uses the term “Trem-spaced” or “Trembucker” to indicate Fender bridge spacing. So if you’re getting a Seymour Duncan JB for example, you want to get the TB-4(Trembucker model) not the SH-4 model.
If you’re buying a humbucker for a Fender guitar(jag-stang, mustang, or any other Fender guitar) get F-Spaced humbuckers or trembuckers.
Here’s a pic of a Fender Cyclone with a regular spaced Seymour Duncan JB in it. You will notice that the pole pieces do not line up with the strings very well. A F-spaced JB would have lined up much better.