PD typically refers to Pitch Diameter in tap terminology. I'm going to guess the other 6 is the number of chamfered or partial-height forming flutes down at the end of the tap which means the tap is a tapered tap. N is probably the type of coating the tap has on it.
Hard to find information on forming tap terminology because every manufacturer seems to use a different labeling system for their taps. Yamawa has some good information
https://www.yamawa.com/en/support/technical/index.html
The 30 lb-ft is the spec that Supertec recommends for the aluminum cases, non-turbo engines
