You can use RDF in the interface and / or in the backend.
As soon as you use it in the interface (RDFa in HTML, separate RDF files, etc.), you play the "part" on the semantic network. There are no websites / applications that could not use RDF.
If you use it only in the backend (database, import of RDF data, etc.), you do not publish any RDF, and therefore there is nothing "semantic" about your site / application. It can consume semantic web data, but it does not produce it.
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