You might see the version listed in your "About" menu as . In Festo’s internal naming convention, "Download 21" refers to the specific hotfix rollup included in the main 4.10 release.
Integrated Modbus TCP and Ethernet support for real-time communication and remote maintenance. festo software tools fst 4.10 download 21
For newer automation projects, Festo has moved toward standardized environments: CODESYS provided by Festo: You might see the version listed in your "About" menu as