Answer:
Both options at the bottom of the "Serviceability" tab of Window 1.1 control whether the design checks are performed according to the analytical or the nonlinear method.
- For the nonlinear serviceability limit state design, it is necessary to specify the provided longitudinal reinforcement.
- The "Use required reinforcement for design of serviceability" option allows you to carry out the SLS design using the amount of reinforcement determined for the ultimate limit state.
More information can be found in the manual of RF‑CONCRETE Surfaces.