You can use the headphones (wired or bluetooth) to control some function of the app, for example during automotive recording you can press a button on the headphones and have a flag on the graph at the time of button press to remember the time when an event occured.
You can use commercial headphones (with buttons) or you can make a custom cable with up to 4 buttons.
Button functions are configurable in HScope.
In HScope Settings (General Settings) you can associate buttons from Keyboard or Headphones buttons to HScope functions.
You can have a simple headphone cable with a single button or can have a console wih up to 4 buttons. Here is the schematic to implement up to 4 buttons with an headphone cable (speakers and microphone are not required). More informations here.