The idea is that a stepper leg with n modifier would create the rights at the square(s) it steps to, not on the square it departs from. So that nR can be used on a royal that cannot pass through check, but is still allowed to make the move out of check. Formally this also makes the final destination of the move an e.p. square, but that doesn't do any harm; normal (replacement) capture can be considered a 'degenerate case' of e.p. capture, where the moved piece still happens to be on the e.p. square. For non-final legs it creates a real e.p. square, though. So for FIDE Pawns the double push should be written fnafmW.
The idea is that a stepper leg with n modifier would create the rights at the square(s) it steps to, not on the square it departs from. So that nR can be used on a royal that cannot pass through check, but is still allowed to make the move out of check. Formally this also makes the final destination of the move an e.p. square, but that doesn't do any harm; normal (replacement) capture can be considered a 'degenerate case' of e.p. capture, where the moved piece still happens to be on the e.p. square. For non-final legs it creates a real e.p. square, though. So for FIDE Pawns the double push should be written fnafmW.