| Mandatory parameter used for creating a recurring payment. It designates the recurring payment rule to be applied. The expected value for this parameter is a chain of characters that comply with the iCalendar (Internet Calendar) specification, described in RFC5545 (see http://tools.ietf.org/html/rfc5545). Among other aspects, this specification allows to define complex recurring payment rules via the RRULE property. For technical reasons, it is not possible to define recurring payment periods that are shorter than one day. The keywords " SECONDLY" / "MINUTELY" /
"HOURLY " are not taken into account. Examples: To program installment payments taking place on the last day of each month for 12 months, the rule is: RRULE:FREQ=MONTHLY;BYMONTHDAY=28, 29, 30, 31;BYSETPOS=-1;COUNT=12 This rule means that if the current month does not have 31 days, the machine will take the 30th into account. If there is no 30th day in a month, the machine will take the 29th into account, and so on until the 28th. Another version of this rule: RRULE:FREQ=MONTHLY;COUNT=5;BYMONTHDAY=-1 - To program installment payments on the 10th of each month for 12 months, the rule is: RRULE:FREQ=MONTHLY;COUNT=12;BYMONTHDAY=10
- To program installment payments every three months up to December 31st, 2016. RRULE:FREQ=YEARLY;BYMONTHDAY=-1;BYMONTH=1, 4, 7, 10;UNTIL=20161231
The installment payments will be due on the first day of January, April, July and October each year. The total number of installments depends on the recurring payment start date (see vads_sub_effect_date parameter). - In order to define a weekly recurring payment to be made every Monday: RRULE:FREQ=WEEKLY;BYDAY=MO
The installments will be made every Monday. Note that the first installment will occur the nearest Monday. - In order to define a weekly recurring payment: RRULE:FREQ=WEEKLY
The installments will occur on the same day if the due date is set to “today”, then every 7 days. - In order to define a recurring payment every two weeks on Monday, with maximum 4 installments: RRULE:FREQ=WEEKLY;INTERVAL=2;COUNT=4;BYDAY=MO
- In order to define a recurring payment every two weeks, on the same day and every 7 days: RRULE:FREQ=WEEKLY;INTERVAL=2;
For more information and examples, visit http://recurrance.sourceforge.net/.
Input and output field, returned in the response (IPN and Return URL). | 
