We have a tag certificates and a pair of tags x509 and x.509. The latter two should obviously be synonyms. What about certificates?
In theory X.509 could be about side topics such as CSR or CRL, but certificates would be appropriate for that anyway. In theory certificates could be encoded differently, but who does that?
In practice I don't see a usage pattern between x.509/x509 and certificates. People seem to use them indifferently.
Previous related discussion: Multiple PKI tags — where we concluded that certificate-authority and pki are different from certificates.