Tags are user-defined labels with which you can categorize events and to dos in BusyCal. You can also search for events in your calendar based on tags.
Note: Tags in BusyCal map to Categories in Exchange/Outlook.
To apply tags to events, tap Add Tags in the Event Details view. Then type a tag and tap Return (or tap the name of a tag you have entered previously), and repeat as desired to add more tags. Tap Done.
Note: The Add Tags option is visible when creating new events or to dos only if you enable it in Event Details Settings.