fontlab opentype features coding

So the code sequence for dlig's goes like this:

sub A A by A_A;
sub K A by K_A;
sub R A by R_A;

I understand why K_A gets priority over A_A, but why A_A gets priority over R_A?
Or vice-versa.

I'm pretty new to font making and I've got FontLab Studio 5, but I'm having trouble understanding all the steps I need to go through to make a lot of the OpenType features work. I understand the concept and I get what most of it means, but it's like I'm missing some pieces to the puzzle.
So are there any books or any tutorials you can suggest that cover the basics of OpenType (hopefully addressing FontLab specifically)?

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