Represents the possible expansion rules used to generate recurrences. From RFC5545:

      The FREQ rule part identifies the type of recurrence rule.  This
      rule part MUST be specified in the recurrence rule.  Valid values
      include SECONDLY, to specify repeating events based on an interval
      of a second or more; MINUTELY, to specify repeating events based
      on an interval of a minute or more; HOURLY, to specify repeating
      events based on an interval of an hour or more; DAILY, to specify
      repeating events based on an interval of a day or more; WEEKLY, to
      specify repeating events based on an interval of a week or more;
      MONTHLY, to specify repeating events based on an interval of a
      month or more; and YEARLY, to specify repeating events based on an
      interval of a year or more.

See rfc5545 for more details.