The checkbox does enable JSX support for the typescript mode of Rule-File-Text. Other than providing JSX it doesn't add additional support for React components. React components can be built with or without JSX. Pegasystems experimented with using React, but initial screen performance on complex call center screens and lower power mobile devices was inferior to other client templating solutions. On some lower power Android devices initial screen render was close to 2X slower. React does offer improved performance on follow-on ajax calls after the shadow DOM is constructed. The modest ajax performance gains didn't compensate for the impact to critical initial screen load scenarios. The investigation was extensive given the level of buzz React has in the developer community and the obvious benefit of the virtual DOM for incremental UI updates. The engineering team performed the evaluation were a bit surprised with the results , but the effort provided solid validation for using Handlebars for rendering Pega 7 UI.
Given Pega's model driven architecture which is now extended to support direct model to client metadata creation the option to cost effectively move to React or other evolving client rendering technologies remains, but alternatives must deliver noticeable performance improvements in all critical application scenarios. The technology used for client rendering has no impact to the model driven UI development experience and runtime behavior or look and feel.
To reduce confusion the label will be changed to just "JSX" in the next Pega 7 release.