Speed sensor magnet numbers

What do the numbers on a speed sensor magnet mean. I have one with a number 3 that works well, but if I put in a number 6, it loses speedometer capability.
Also, I see a lot of magnets for sale, but never see one offered with a specific number on it.
Any info would be appreciated.

My magnet has the number 4 and it has 4 poles :smiley: