Assigned calendars are your Round Robin calendars where you can have users assigned to them. Unassigned calendars are your Event calendars that cannot have users assigned to them.
Assigned Calendars
Typically, you would use a Round Robin calendar when you have more than one person actioning or managing appointments on a single calendar.
The availability for this calendar will be based on the availability set in this calendar's settings as well as the availability of the individual/s assigned to the calendar.
It will also take into account any blocked off time an individual has in their Google/Outlook calendar if they have synced this within their settings. You can learn how to sync your Google calendar here or your Outlook calendar here.
Appointment distribution can be used to determine how different appointments are delegated and prioritised amongst the users assigned to the calendar. You can find out more about appointment distribution here.
If there are multiple users assigned to a calendar and they both have availability at the same time, multiple appointments can be booked at the same time with the individual users. The system will look at the number assigned to the Maximum Bookings Per Slot (Per User) setting. Once the maximum number of bookings have been made for a particular time slot, then the booking will either be assigned to the next user available within the round robin queue, or that time will become unavailable if there are no more users that can take appointments in that particular time slot.
Unassigned Calendars
Typically, you would use an Event calendar if you are managing a definitive appointment schedule that one or multiple users take turns at managing and booking appointments for. With unassigned calendars, you cannot have the system distribute appointments to your users and you can only have it linked to one specific Google or Outlook calendar to block out or determine the availability around your general calendar office hours.
The availability for this calendar will be based on the availability set in this calendar's settings.