+=head3 RECURRING BILLING FIELDS
+
+=over 4
+
+=item * interval
+
+Interval expresses the amount of time between billings: digits, whitespace
+and units (currently "days" or "months" in either singular or plural form).
+
+=item * start
+
+The date of the first transaction (used for processors which allow delayed
+start).
+
+=item * periods
+
+The number of cycles of interval length for which billing should occur
+(inclusive of 'trial periods' if the processor supports recurring billing
+at more than one rate)
+
+=back
+