The history of some chess positions can be determined with certainty and some cannot.
PRA looks at each alternative history and finds their different solutions. These typically revolve around if the right to castle exist, then you have some solution; or if last move allows en-passant then another solution exists.
RS involves 'mutual dependency', for ...
Just to clarify IA Petr Harasimovic's comment in the answer provided by Cleveland, here is a position in which capturing en passant is illegal due a diagonal pin.
[FEN "5k2/2p2b2/8/3P4/8/2K5/8/8 w - - 0 1"]
1. Kc4 c5