What does the BusyCal Menu app do?
BusyCal Menu is a helper app that lives in the macOS menu bar, allowing you to quickly view your calendar, upcoming events, and alarms.
You can open the menu as a popover from the menu bar icon, or detach it into a floating window that you can keep on screen while working.
The Menu app can use calendar data stored on your Mac after BusyCal quits, but it does not sync accounts itself. Keep the main BusyCal app running to sync with Outlook, iCloud, and other services. You can close its window with Command-W or the red close button while it continues syncing.