I have an article forthcoming in an upcoming issue of "Livebearers" that goes into detail about Endlers and covers the classification system developed by many people including John Endler, Armando Pou, AdrianHD and others. It's either the next issue or the issue right after that.
For class N, it's basically traceability. I can trace the chain of possession of these endlers back to the man who collected them in the wild. In my case: Me--AdrianHD, breeder--Armando Pou, collector.
The classification system is necessary because hybrids are often difficult to distinguish, and hybridization (and thus dilution of the species) is rampant to the point that almost all Endlers you find in stores are hybrids being sold as "Endlers" without disclosing that (and probably often without even knowing that) they are crossed with guppies.
Below is from Endlers U.S.A. website
(Class N): Any Endler's Livebearer (and progeny) that can be shown to have originated from their native waters in Venezuela will be considered a 'Class N' Endler.
(Class K):The progeny of any Endler's Livebearer crossed with any other livebearer or any Hybrid strain will be considered a 'Class K' Endler.
(Class P)Class P: Any fish of unknown origin but appearing to be an Endler's Livebearer based on the characteristics of size, shape and color will be considered a 'Class P' Endler.