From the new macOS Human Interface Guidelines:
By default, the right side of the Touch Bar displays an expandable Control Strip that includes controls for performing system-level tasks such as invoking Siri, adjusting the brightness of the main display, and changing the volume level. Previously, users accessed most of these controls with a top row of physical function keys. You can implement app-specific controls in the app region to the left of the Control Strip. An Esc (Escape) button or other system-provided button may appear to the left of the app region, depending on context.
