A,B, P etc

[Rumata]

Does anyone know or have a link that explains what exactly the values for A,B, F, P etc represent in a sight reduction?

 I was asked recently and I know anything with H has to do with altitudes and the Zs are azimuths but the rest I don't know with enough certainty to pass on.

Thanks

BigBill, maybe it refers to Ageton method ? Or there is another one, just called S-tables method which used smartly developed tables and can be used for any latitude.
Also, in Dutton, composite sailing. Another remote possibility is the analysis of deviation. It includes 6 coefficients: A,B,C,D E,J. See Bowditch for details. Unless those letters represent angles and sides of the spherical triangles ( which I think they are), I don't know any other instance where I saw those letters regrading sight reduction.