You're seeing just the callback week_of_month, go back to Cldr.Calendar module for more information.
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week_of_month(arg1, week, arg3)

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week_of_month(Calendar.year(), week(), ::
  {Calendar.month(), week()} | {:error, :not_defined}

Returns a tuple of {month, week_in_month} for a given year, month or week, and day for a a calendar.

The week_in_month is calculated based upon the calendar configuration.