The idea of an App Specific Button is that in many applications there are one or two actions that you perform quite often. For example, in my workflow, in email it is making an email as spam, and in Slack it is advancing to the next channel with unread messages, etc.

Each of these normally takes multiple key presses (the hotkey for spam is exclamation point which is shift-1, and the hotkey sequence for advancing to the next channel with unreads in Slack is command-k and then Enter). The left and right App Specific Buttons are programmed to be the 2 most common such actions in each application. You can use my macros as inspiration, but you’ll want to customize these based on your personal workflow.