Calendar management : which Calendar is used by whom?...
At least there is one Calendar, the General Calendar (created during the installation).
It is possible to create a Calendar for a Management Unit, as well as a Calendar for all local Management Units of the same Type.
It is also possible to create Template Calendars and then to distribute them to MU Types or MUs.
How to recognize which Calendar is used by a Task?
Just check on which MU the Task is created. If there is a Calendar for this MU, this is the one. If not, if there is a Calendar for the corresponding MU Type this is the one.
In all other cases, Tasks use the General Calendar.
But beware :
Check MU Calendar -> MU Type Calendar -> General Calendar is not global, it is made year by year.
If for a Task running against MU "ABC", the Calendar for the MU "ABC" covers 2000-2008, the Calendar for the MU Type "A" covers 2000-2008 and 2010-2012 and the General Calendar covers 2000-2015, the Task will use the MU "ABC" Calendar from 2000 to 2008, the General Calendar in 2009, the MU Type "A" Calendar from 2010 to 2012, and the General Calendar again from 2013 to 2015!
It is therefore recommended to define all Calendars for the same period, except for very specific needs.