Answer:
In cross-section design checks, RF‑CONCRETE Members or CONCRETE calculates with the stress-strain diagram for reinforcing steel according to DIN EN 1992‑1‑1, 3.2.7(2)a; that is, with an increasing upper branch up to ftk and a strain limitation of εud = 0.025.
In RF‑CONCRETE Members or CONCRETE, there is no option to switch to a design with a horizontal branch according to 3.2.7(2)b.
However, if necessary, you can calculate with a user-defined material, in which ftk = fyk.