On this technical help page:
there is a paragraph about what’s the difference between Hardware version, Firmware version, and App version.
“from firmware 2.1.9, only OS 2.3 (and above) will be supported” — this sentence means: firmware 2.1.9 and future firmwares will only support hardware version 2.3 and above (i.e. hardware 2.3, 3.0, 3.1, 3.2 etc.). You have OS 2.3, which means OpenSprinkler 2.3, i.e. Hardware version 2.3, that means your controller can be upgraded to firmware 2.1.9 and future firmwares.
App refers to Mobile app, you can always update your app in the app store, this is regardless of what hardware or firmware version you have. You don’t even need an OpenSprinkler to install the mobile app. Firmware is the software running on the controller, it is different from the mobile app. Mobile app can be thought of as the ‘front end’ or ‘graphics user interface’ of the controller.