OWL2DL ontology is a graph or a set of axioms?

Hi,Please, I want to check some points:1/can we consider OWL2DL ontology as set of axioms? Or does the OWL2DL ontology document is it a set of axioms2/Most of works in litterature consider it as graph? Are they right?3/ According the OWL2  structural specification, is it considered as a set of axioms or graph?4/ Please, if I will work on OWL2DL ontology,:    -Must I refer  essentially to OWL 2 Web Ontology Language Direct Semantics (Second Edition)?
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| OWL 2 Web Ontology Language Direct Semantics (Second Edition)OWL 2 Web Ontology Language Direct Semantics (Second Edition) W3C Recommendation 11 December 2012 This version: http://www.w3.org/TR/2012/REC-owl2-direct-semantics-20121211/ Latest version (series 2):  |
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   - Have I to use Direct Model-Theoretic Semantics for OWL 2 in order to introduce the OWL2DL ontology (in my papers)?Regards and thanks in advance.