{
  "formatName": "FAB2 Book Container",
  "version": "0.2",
  "formatDescription": "A container for digital books that stores the book as typographic data, unlike other alternatives that store books as web documents inside an archive. The container has no scripting and no executable content. Page records carry resolved line geometry, so line breaks and optical margins are decided once when the book is typeset rather than recomputed on the reading device. The signature contains an initial 16 byte magic byte header, after which it holds two little-endian unsigned 32-bit lengths, a Zstandard-compressed JSON manifest, and a chunk arena containing one compressed page block and the book's assets.",
  "binaryFileFormat": null,
  "formatTypes": "Page Description, Word Processor",
  "identifiers": [
    {
      "identifierText": "fmt/2108",
      "identifierType": "PUID"
    },
    {
      "identifierText": "application/vnd.fabylon.book",
      "identifierType": "MIME"
    }
  ],
  "internalSignatures": [
    {
      "signatureID": 4473,
      "name": "FAB2 Book Container",
      "note": "BOF; Offset: 0; Magic Bytes: 'ASCII FABYLON_BOOK_02!'",
      "byteSequences": [
        {
          "positionType": "Absolute from BOF",
          "offset": 0,
          "maxOffset": 0,
          "byteSequence": "464142594C4F4E5F424F4F4B5F303221",
          "endianness": null,
          "byteSequenceID": 4869
        }
      ],
      "provenance": "The signature is fixed and always at offset 0 with no preceding bytes. The generation number is inside the magic string itself, so an unknown future generation can be rejected before parsing. An earlier generation used the signature FABYLON_REALM_01 (hex 46 41 42 59 4C 4F 4E 5F 52 45 41 4C 4D 5F 30 31) with a different internal structure; it is still readable by current software but is no longer written."
    }
  ],
  "externalSignatures": [
    {
      "externalSignature": "fab",
      "signatureType": "File extension"
    }
  ],
  "relationships": [],
  "containerSignatures": [],
  "formatNote": "Specification: https://www.google.com/url?q=https://fabylon.net/fab/specification/&source=gmail&ust=1787034048114000&sa=E Related software: Fabylon Bookmaker \u2014 writes .fab \u2014 https://www.google.com/url?q=https://www.martijnbenders.nl/bookmaker/&source=gmail&ust=1787034048114000&sa=E Fabylon Reader \u2014 reads .fab \u2014 https://www.google.com/url?q=https://fabylon.net/fabreader/&source=gmail&ust=1787034048114000&sa=E",
  "lastUpdatedDate": "02/09/2026",
  "formatSourceDate": "02/09/2026",
  "fileFormatID": 4984,
  "supportedBy": [
    365
  ],
  "developedBy": [
    365
  ],
  "submittedBy": [
    365
  ]
}