Rodinal is a concentrated developer containing p-Aminophenol. It's normally used diluted between 1+25 to 1+100. So in this case it's used in combination with Hydroquinone.

Using 2 developing agents together give additivity, usually it's Metol or Phenidone & Hyroquinone/Glycin/Pyrocatechin, but p-Aminophenol can be used instead and in this developer gives very fine grain which in a print causes greater warmth.

Ian