Since this question seems to be coming up regularly. I thought I would try to understand how exactly it appears in PMBOK and what is the trail here that we are missing.
1. PMBOK fails to clearly specify how Resource calendar is created the first time. Because that is the main question right. We know it is created as an output of Acquire Resources process. But then it is also an input to the same process.
Adding more to the confusion, it is also an input to the Estimate Activity Resources process.
One possible explanation to this is, it is an iterative process so inputs and outputs keep getting modified.
2. After all the discussion, it somehow does not make sense how exactly it gets created OR where does the information come from, which will be mentioned later in Resource Calendar.
For me, it would make some sense that it actually comes from EEF, in terms of database or information. All the information related Personnel either comes from EEF or gets updated as an output in EEF. If we look at the content of the resource calendar --> shifts, holidays, working days, start-end of business hours, possible parallel assignments on other projects, geographic location constraints.
I would be glad if someone else can also comment on this point of view.