I don't know the difference! But in fontlab when I open a font, in the kern feature I can see that LTR kerning is interpreted like this:
pos uni0631 uni0628 -225;
But RTL kerning is interpreted like this:
pos uni0631 uni0628 <-225 0 -225 0>;
So I suppose RTL kerning should be implemented with adjustments applied to position of the glyph too, maybe I'm wrong; I'm not a pro. (see AFDKO OpenType Feature File Specification)
About the kerning in RoboFont, I think there is a malfunction in the "Right to left" Option in the kerning panel. Here is what I do:
I open a font which already has RTL kerning, Robofont interprets kerning correctly. I open the kerning panel and click on a class pair, the kerning shows up but order of glyphs is incorrect, I turn the "Right to Left" option on; Nothing Happens! I turn it off, now order of glyphs becomes right, even so the kerning has been implemented in RTL order.