BOOK_BACKEND_PROPERTY_PREFER_VCARD_VERSION

A book backend string property name containing a vCard version the backend prefers.

Common values are "3.0" and "4.0". It is only what the backend prefers for the exchange, it can receive other vCard versions too, the same as it can return vCard data in a different version (like when the server has stored vCards in multiple versions, then they are not converted by the backend.