You're seeing just the callback plus, go back to Cldr.Calendar module for more information.
Link to this callback

plus(year, month, day, months_or_quarters, increment, options)

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  year :: Calendar.year(),
  month :: Calendar.month() | week(),
  day ::,
  months_or_quarters :: :months | :quarters,
  increment :: integer(),
  options :: Keyword.t()
) :: {Calendar.year(), Calendar.month(),}

Increments a Date.t or Date.Range.t by a specified positive or negative integer number of periods (year, quarter, month, week or day).

Calendars need only implement this callback for :months and :quarters since all other date periods can be derived.