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It's the number of poles in the alternator plus...
The way that the "diesel tachs" that run off the alternator work is based primarily on the number of poles that are in the tach - usually 12. A lot - but not all - of these tachs have one or two rotary switches on the back; typically on is for selecting the number of poles since not all alternators have 12 poles. The other rotary switch is for selecting the pulley ratio. Sometimes both functions are in a single switch. Calibration is done either by having another tach (mechanical tach for example) hooked up and then trying different switch settings to get it to register close to the "mechanical" tach; alternatively use a "guesstimate" method - if your diesel idles at 800rpm according to spec, adjust the tach so it shows 800rpm, if peak speed at spec if 3200 rpm - go to WOT and see if tach is close to 3200rpm.
Learned about this the hardway installing a tach on my little Universal diesel. Good luck, I hope that this helps.