In non-technical terms: card10 will have had what I am going to call apps in this text. Apps are small programs which can be run on card10. Examples would be:
A watch-face showing the current time & data about the environment (temperature, humidity, etc)
A small game
An ECG diagram
There can only ever be one app running at a time. All apps are stored on the external flash where they can be copied to via either Bluetooth or using USB. When card10 starts up, one of these apps will be started. This is the "default app".
If no default app was found, card10 will go straight to the menu. Otherwise, pressing the power/reset-button at any time, will stop the currently running app and take the user to the menu.
The menu allows the user to select which app to run next. How this works is open for discussion. Any kind of menu would work. This ranges from a very simple file-list to an animated menu with icons for each app. To interact with the menu, the two buttons on the right and the lower button on the left can be used. If favored, we could probably also use the ECG pad below the display as a button.
The menu also needs one other functionality: It needs an option to open the USB-Device so new apps can be loaded onto the card10. How this option is integrated into the UX is open for discussion. Activating the USB-Device is similar to an app: To get back to the menu, the user needs to press the power/reset-button once again.
To put it into a concrete question: What will card10's menu have looked like and how will it have worked? What buttons would I need to have pressed to navigate around the menu?