[KevinH]

Originally Posted by Doitsu

If you're using my Sigil plugin, you'll need to add/edit this line in EpubCheck.json.

  "usage": true,

Or if you're using the command line:

java -jar "C:\Users\user\AppData\Local\sigil-ebook\sigil\plugins\EpubCheck\epubcheck.jar" "C:\Users\user\Downloads\backlink.epub" -u --json -

The output will be:

Spoiler:

No errors or warnings detected.
{
  "messages" : [ {
    "ID" : "OPF-088",
    "severity" : "USAGE",
    "message" : "Unrecognized epub:type value \"backlink\".",
    "additionalLocations" : 0,
    "locations" : [ {
      "url" : {
        "opaque" : false,
        "hierarchical" : true
      },
      "path" : "OEBPS/Text/Section0001.xhtml",
      "line" : 12,
      "column" : 116,
      "context" : null
    } ],
    "suggestion" : null
  } ],
  "customMessageFileName" : null,
  "checker" : {
    "path" : ".\\Downloads\\backlink.epub",
    "filename" : "backlink.epub",
    "checkerVersion" : "5.2.1",
    "checkDate" : "06-21-2025 18:57:32",
    "elapsedTime" : 32994,
    "nFatal" : 0,
    "nError" : 0,
    "nWarning" : 0,
    "nUsage" : 1
  },
  "publication" : {
    "publisher" : null,
    "title" : "backlink test",
    "creator" : [ ],
    "date" : "2025-06-21T18:54:55Z",
    "subject" : [ ],
    "description" : null,
    "rights" : null,
    "identifier" : "urn:uuid:15d02007-6607-431f-be22-581de695ebdf",
    "language" : "en",
    "nSpines" : 2,
    "checkSum" : 0,
    "renditionLayout" : "reflowable",
    "renditionOrientation" : "auto",
    "renditionSpread" : "auto",
    "ePubVersion" : "3.3",
    "isScripted" : false,
    "hasFixedFormat" : false,
    "isBackwardCompatible" : false,
    "hasAudio" : false,
    "hasVideo" : false,
    "charsCount" : 218,
    "embeddedFonts" : [ ],
    "refFonts" : [ ],
    "hasEncryption" : false,
    "hasSignatures" : false,
    "contributors" : [ ]
  },
  "items" : [ {
    "id" : "Section0001.xhtml",
    "fileName" : "OEBPS/Text/Section0001.xhtml",
    "media_type" : "application/xhtml+xml",
    "compressedSize" : 280,
    "uncompressedSize" : 456,
    "compressionMethod" : "Deflated",
    "checkSum" : "bb2932f84166f7ae12b1c71e4918fbf18996d1aadbad4dc943bcfd71bee634",
    "isSpineItem" : true,
    "spineIndex" : 0,
    "isLinear" : true,
    "isFixedFormat" : false,
    "isScripted" : false,
    "renditionLayout" : "reflowable",
    "renditionOrientation" : "auto",
    "renditionSpread" : "auto",
    "referencedItems" : [ "OEBPS/Text/Section0001.xhtml#en1" ]
  }, {
    "id" : "ePubCheck.NoManifestRef:META-INF/container.xml",
    "fileName" : "META-INF/container.xml",
    "media_type" : null,
    "compressedSize" : 180,
    "uncompressedSize" : 265,
    "compressionMethod" : "Deflated",
    "checkSum" : "83e412f1ac7d52a9f73d82d26c9ce241591c60b8e9c384956e9e588e1d5fc",
    "isSpineItem" : false,
    "spineIndex" : null,
    "isLinear" : false,
    "isFixedFormat" : null,
    "isScripted" : false,
    "renditionLayout" : null,
    "renditionOrientation" : null,
    "renditionSpread" : null,
    "referencedItems" : [ ]
  }, {
    "id" : "ePubCheck.NoManifestRef:OEBPS/content.opf",
    "fileName" : "OEBPS/content.opf",
    "media_type" : null,
    "compressedSize" : 428,
    "uncompressedSize" : 912,
    "compressionMethod" : "Deflated",
    "checkSum" : "afd0bacff9dda1cae13c6b2ba5952345adc4c26fbee84ef84236cd2d3e3ea8",
    "isSpineItem" : false,
    "spineIndex" : null,
    "isLinear" : false,
    "isFixedFormat" : null,
    "isScripted" : false,
    "renditionLayout" : null,
    "renditionOrientation" : null,
    "renditionSpread" : null,
    "referencedItems" : [ ]
  }, {
    "id" : "ePubCheck.NoManifestRef:mimetype",
    "fileName" : "mimetype",
    "media_type" : null,
    "compressedSize" : 20,
    "uncompressedSize" : 20,
    "compressionMethod" : "Stored",
    "checkSum" : "e468e350d1143eb648f6c7bbd63111ec544a361ca74ecef256ac91f48b",
    "isSpineItem" : false,
    "spineIndex" : null,
    "isLinear" : false,
    "isFixedFormat" : null,
    "isScripted" : false,
    "renditionLayout" : null,
    "renditionOrientation" : null,
    "renditionSpread" : null,
    "referencedItems" : [ ]
  }, {
    "id" : "nav.xhtml",
    "fileName" : "OEBPS/Text/nav.xhtml",
    "media_type" : "application/xhtml+xml",
    "compressedSize" : 368,
    "uncompressedSize" : 741,
    "compressionMethod" : "Deflated",
    "checkSum" : "c4e8b5e345a9ab3c2b6dc8fa54253309213d2fab9dbb322abcf4cfd441",
    "isSpineItem" : true,
    "spineIndex" : 1,
    "isLinear" : false,
    "isFixedFormat" : false,
    "isScripted" : false,
    "renditionLayout" : "reflowable",
    "renditionOrientation" : "auto",
    "renditionSpread" : "auto",
    "referencedItems" : [ "OEBPS/Styles/sgc-nav.css", "OEBPS/Text/Section0001.xhtml", "OEBPS/Text/nav.xhtml#toc" ]
  }, {
    "id" : "sgc-nav.css",
    "fileName" : "OEBPS/Styles/sgc-nav.css",
    "media_type" : "text/css",
    "compressedSize" : 151,
    "uncompressedSize" : 275,
    "compressionMethod" : "Deflated",
    "checkSum" : "d7bc6622fc698f2beeaeea31a6f4123de3d71536ca69c6673a1e28f2064c372",
    "isSpineItem" : false,
    "spineIndex" : null,
    "isLinear" : false,
    "isFixedFormat" : null,
    "isScripted" : false,
    "renditionLayout" : null,
    "renditionOrientation" : null,
    "renditionSpread" : null,
    "referencedItems" : [ ]
  } ]
}

We should file an official bug report with epubcheck to update its epub3 structural semantics vocabulary to the very latest 1.1 standard just released in March.

Does the info also barf when epub:type="backlink" is replaced by much older epub:type="referrer"?