Answer:
RF-/LTB requires explicit internal forces over the member axis for the design. This condition is due to various standard requirements that consider the member element to be designed as a structural component, and use result gradients over the member axis to determine the design parameters (axial force distributions, bending moment distributions, and so on).
In order to meet this requirement, the internal forces to be designed must project explicit design cases for the analysis core. Load cases and load combinations are representatives of such explicit design cases. Since result combinations are usually used for the enveloping results, they are locked in RF‑/LTB.
