The iCalendar [RFC5545] data format is used to represent calendar data and is used with the iCalendar Transport-Independent Interoperability Protocol (iTIP) [RFC5546] to handle scheduling operations between calendar users. iCalendar is in widespread use, and in accordance with provisions in that specification, extension elements have been added by various vendors to the data format in order to support and enhance capabilities. This specification collects a number of these ad hoc extensions and uses the new IANA registry capability defined in [RFC5545] to register standard variants with clearly defined definitions and semantics. In addition, some new elements are introduced for features that vendors have recently been requesting.

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