The log:semantics of a document is the formula.
achieved by parsing representation of the document.
For a document in Notation3, log:semantics is the
log:parsedAsN3 of the log:contents of the document.
For a document in RDF/XML, it is parsed according to the
RDF/XML specification to yield an RDF formula
(a subclass of N3 log:Formula).
[Aside: Philosophers will be distracted here into worrying about the meaning
of meaning. At least we didn't call this function "meaning"!
In as much as N3 is used as an interlingua for interoperability
for different systems, this for an N3 based system is the meaning
expressed by a document.]
(Cwm knows how to go get a document and parse N3 and RDF/XML
it in order to evaluate this.
Other languages for web documents
may be defined whose N3 semantics are therefore
also calculable, and so they could be added in due course.
See for example GRDDL, RDFa, etc)