Both CALENDAR & CALENDARAUTO functions return single column named "Date" that contains set of dates.
In CALENDAR function, the range of date is from the specified start date to the specified end date. On the other hand CALENDARAUTO function, the range of date is calculated automatically based on data in the model.
CALENDAR
https://docs.microsoft.com/en-us/dax/...
CALENDARAUTO
https://docs.microsoft.com/en-us/dax/...