22.214.171.124.8 Calendar Items and Meeting Messages
A Message object is a calendar item when the value of the PidTagMessageClass property ([MS-OXCMSG] section 126.96.36.199) starts with "IPM.Appointment." or equals "IPM.Appointment". A Message object is a meeting message when the value of the PidTagMessageClass property starts with "IPM.Schedule.Meeting.". Clients SHOULD create items of these types with a best body format type of RTF. Clients can use a plain text body instead but SHOULD NOT create calendar items or meeting messages with a best body format type of HTML.
Each of the leaf MIME entities specified in this section SHOULD use the UTF-8 form as its character set (2), as specified in [RFC2445]. A MIME writer MAY<70> enable a user or a system administrator to select a character set (2) encoding other than the UTF-8 form for the MIMEheaders (2) and MIME entities generated from the properties of a Message object. The iCalendar message body, however, always remains encoded in the UTF-8 form.