Any property name with a "X-" prefix
This class of property provides a framework for defining non-standard properties.
The default value type is TEXT. The value type can be set to any value type.
IANA, non-standard, and language property parameters can be specified on this property.
This property can be specified in any calendar component.
The MIME Calendaring and Scheduling Content Type provides a "standard mechanism for doing non-standard things". This extension support is provided for implementers to "push the envelope" on the existing version of the memo. Extension properties are specified by property and/or property parameter names that have the prefix text of "X-" (the two-character sequence: LATIN CAPITAL LETTER X character followed by the HYPHEN- MINUS character). It is recommended that vendors concatenate onto this sentinel another short prefix text to identify the vendor. This will facilitate readability of the extensions and minimize possible collision of names between different vendors. User agents that support this content type are expected to be able to parse the extension properties and property parameters but can ignore them.
At present, there is no registration authority for names of extension properties and property parameters. The value type for this property is TEXT. Optionally, the value type can be any of the other valid value types.
This property is defined by the following notation:
x-prop = x-name *(";" icalparameter) ":" value CRLF
The following might be the ABC vendor's extension for an audio-clip form of subject property: