fokof has pretty much explained it.
The GK-3B hex pickup is split internally into two x three pole pickups, adjustable to suit different string widths of different basses.
The white markers indicate the centre of the pickup middle pole, and is best aligned with the "E" string and the "G" string, loosen the holding screws first, (I broke one marker off mine), before using the marker peg to adjust the pickup width offset, or alternatively use the screwdriver still seated in the locking screw.
The indent mark in the middle of the pickup needs to align exactly between the "A" and "D" strings.
Odd thing about the cable into the GK-3B is it comes out the middle-rear of the pickup, where the guitar GK-3 cable runs out the end of the pickup.