What RORB does is that it averages the losses from the different types of surfaces over the subarea and then applies the calculated average loss based on the proportion of the subarea that is DCIA, ICIA and pervious area. RORB does not calculate them separately.
This works in the same way as the original pervious and impervious fractions functionality that was already in RORB. The new DCIA and ICIA just allows the user to have a third surface and to specify the initial and continuing losses instead of using the default 0.9 runoff coefficient for the impervious areas.
This is explained in more detail for the original impervious/pervious functionality in the RORB manual Section 3.4.2.