Assuming it is for a Yanmar.
Picture of original Hitachi alternator not your Bosch replacement
The extra wire is the W for a rev counter (should have an inline fuse)
What andsarkit says. ................
The tacho connection on Hitachi alternators is "P"
L is the warning light connection
R is the excitation connection which should have 12 v applied from the "ignition" switch when in the run position