You can assign an item to a standard category, such as Family or Travel, or to a category that you create. Because categories are color-coded, when you view the item list, it's easy to see which items belong to which category. You can organize your messages, events, contacts, and other items (items: Units of information in Outlook, such as e-mail messages, calendar events, contacts, tasks, and notes.) by assigning them to categories. Yes creating an appointment is essential to have a reminder to work on an email with a deadline! So please add this.

Users of Outlook for Mac 2016 are not seeing what they're supposed to see.

By marking a calendar item (appointment, meeting, or event) as Private, you hide the details of the item from people who have permission to view your calendar.