Does anyone know the difference between OpenType Pro vs OpenType ProN fonts?
Apple installs Japanese fonts such as: Hiragino Kaku Gothic Pro W3.otf
They also install Hiragino Kaku Gothic ProN W3.otf
In terms of the Hiragino fonts that Apple bundles with Mac OS X, the difference between Pro and ProN is about glyph preference. The former are JIS90-savvy fonts, meaning that the JIS90 glyphs are the default in the 'cmap' table. The latter are JIS2004-savvy fonts, meaning that the JIS2004 glyphs are the default. This difference is manifested as subtle glyph differences in approximately 170 kanji.
I notice Adobe also sells several Japanese fonts with the "N" ending, such as Kozuka Gothic Pr6N. Iin that case, does the Pr stand for Pro? What does the 6 stand for? The N?
The Pr is an abbreviated form of Pro. The 6 stands for the /Supplement of the glyph set, which is represented in the final digit of the glyph set name, specifically Adobe-Japan1-6. The N, like with the Hiragino fonts bundled with Mac OS X, means that the font is JIS2004-savvy.
Technically, the Hiragino fonts bundled with Mac OS X should use the Pr5 or Pr5N identifier, because they use the Adobe-Japan1-5 glyph set. Apple chose to simply use Pro, but they did adopt the convention of using N to identifiy JIS2004-savvy fonts. In fact, Apple was involved in establishing this convention.
I see there is also a Ryo Gothic PlusN. What is the Plus for?
The Plus is considered a synonym for Std, which, for OpenType Japanese fonts, means that the font is based on the Adobe-Japan1-3 or earlier glyph set. We used Plus because the fonts start with the Ryo kana fonts that we developed, then added glyphs from Kozuka Gothic or Kozuka Mincho to fill out the glyph set to complete Adobe-Japan1-3 coverage.
I hope this helps...