RDF/XML is a serialization format that can be used to serialize ANY graph of RDF statements, not just OWL ontologies. It has no specific support for OWL ontologies and is therefore considered to be hard to read and understand.https://www.w3.org/TR/rdf-syntax-grammar/

Quote from the W3C standard document about OWL:

The primary exchange syntax for OWL 2 is RDF/XML [RDF Syntax]; this is indeed the only syntax that must be supported by all OWL 2 tools (see Section 2.1 of the OWL 2 Conformance document [OWL 2 Conformance]).
While RDF/XML provides for interoperability among OWL 2 tools, other concrete syntaxes may also be used. These include alternative RDF serializations, such as Turtle; OWL/XML; and a more "readable" syntax, called the Manchester Syntax [OWL 2 Manchester Syntax], that is used in several ontology editing tools. Finally, the functional-style syntax can also be used for serialization, although its main purpose is specifying the structure of the language [OWL 2 Structural Specification].

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