Component/Property Presence
------------------- ---------------------------------------------
METHOD 1 MUST be "DECLINECOUNTER"
VTODO 1
ATTENDEE 1+ MUST for all attendees
DTSTAMP 1
ORGANIZER 1
SEQUENCE 1 MUST echo the original SEQUENCE number
UID 1 MUST echo original UID
ATTACH 0+
CATEGORIES 0 or 1 This property may contain a list of values
CLASS 0 or 1
COMMENT 0 or 1
CONTACT 0+
CREATED 0 or 1
DESCRIPTION 0 or 1
DTSTART 0 or 1
DUE 0 or 1 If present DURATION MUST NOT be present
DURATION 0 or 1 If present DUE MUST NOT be present
EXDATE 0+
EXRULE 0+
GEO 0 or 1
LAST-MODIFIED 0 or 1
LOCATION 0 or 1
PERCENT-COMPLETE 0 or 1
PRIORITY 0 or 1
RDATE 0+
RECURRENCE-ID 0 or 1 MUST only if referring to an instance of a
recurring calendar component. Otherwise
it MUST NOT be present.
RELATED-TO 0+
REQUEST-STATUS 0+
RESOURCES 0 or 1 This property MAY contain a list of values
RRULE 0+
STATUS 0 or 1 MAY be one of COMPLETED/NEEDS ACTION/IN-
PROCESS
URL 0 or 1
X-PROPERTY 0+
VTIMEZONE 0+ MUST be present if any date/time refers to
a timezone
X-COMPONENT 0+
VALARM 0
VEVENT 0
VFREEBUSY 0