What is a resource calendar?

A resource calendar is the calendar of a mailbox for which the parameter "AutomateProcessing" has been set to "AutoAccept".

Because user mailboxes use a fixed value of "AutoUpdate" for this parameter, user mailbox calendars can never be resource calendars.

What makes a resource calendar different is the ability to set specific parameters on the containing mailbox that governs the handling of meeting items. By default, for example, a resource calendar will replace the subject of a meeting with the organizer's name and delete attachments and notes of the item stored in its calendar.

A description of these and other options can be found in the Parameters section of the Set-CalendarProcessing section in Microsoft's documentation.