This refers to the mounting ‘legs’ of a humbucker. While for many guitars, it doesn’t matter (most use short legs), longer legs might be chosen if your guitar has an arched top (like a Les Paul). Traditionally, arch-top guitars used long leg pickups, but these days short legs are more common in every guitar. The Seymour Custom Shop can use either on your pickup.
Short Mounting Legs
Long Mounting Legs