Complex sentence: If the rook on d3 moves to d5, it is possible to disambiguate with either Rdd5 or R3d5, but the file takes precedence over the rank, so Rdd5 is correct.
Split into simpler sentences: If the rook on d3 moves to d5, it is possible to disambiguate with either Rdd5 or R3d5. In cases such as these the file takes precedence over the rank, so Rdd5 is correct.