When a whole body is irradiated uniformly only factor "W" R is used, but if the irradiation of a body is partial or non-uniform the factor "W" T is also used to calculate dose to each organ or tissue.
When a whole body is irradiated uniformly only the radiation weighting factor "W" R is used, and the effective dose equals the whole body equivalent dose. But if the irradiation of a body is partial or non-uniform the tissue factor "W" T is used to calculate dose to each organ or tissue.