2D Result Lines were first implemented in Structural Designer release 2019 (released Mar 2019) allowing the engineer to investigate sectional forces around openings in concrete walls for example. Subsequent releases have continually enhanced this feature, adding interactive concrete section design in release 2019 SP1 and enhanced design forces in release 2019 SP2.
See this video demonstrating result lines in action.
The Tekla Structural Designer 2020 release further enhances the Interactive design using Result lines feature to add the saving of both the reinforcement and manually added/adjusted design forces defined in the interactive design dialog.