Meaning of precondition DAV:supported-live-property

In our group we are discussing about the exact meaning of the additional
precondition(s) DAV:supported-live-property for PROPFIND and PROPPATCH (3.11
and 3.12 in RFC 3253). In particular, the meaning of "the semantics of a
property being supported by the server".

Is the following an example?
A PROPPATCH request intends to modify the value of a "checkout-fork"
property of a checked-out VCR to something different than DAV:forbidden or
DAV:discouraged, say, to an empty value. Then the
DAV:supported-live-property precondition is violated.

Another interpretation is, that it has to do with properties which are not
REQUIRED (...but there are not such properties as fas as I can see!).

Thanks,
Peter