"D7" meaning in Conventions internationales

{
  "categories": [
    {
      "kind": "other",
      "name": "Symboles en conventions internationales",
      "parents": [],
      "source": "w"
    },
    {
      "kind": "other",
      "name": "Conventions internationales",
      "orig": "conventions internationales",
      "parents": [],
      "source": "w"
    }
  ],
  "lang": "Conventions internationales",
  "lang_code": "conv",
  "pos": "symbol",
  "pos_title": "Symbole",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "Lexique en conventions internationales de l’imprimerie",
          "parents": [],
          "source": "w"
        }
      ],
      "glosses": [
        "Code ISO 216 du format de papier de 96 mm × 68 mm."
      ],
      "id": "fr-D7-conv-symbol--~oyF9Z4",
      "raw_tags": [
        "Imprimerie"
      ]
    }
  ],
  "word": "D7"
}

{
  "categories": [
    "Symboles en conventions internationales",
    "conventions internationales"
  ],
  "lang": "Conventions internationales",
  "lang_code": "conv",
  "pos": "symbol",
  "pos_title": "Symbole",
  "senses": [
    {
      "categories": [
        "Lexique en conventions internationales de l’imprimerie"
      ],
      "glosses": [
        "Code ISO 216 du format de papier de 96 mm × 68 mm."
      ],
      "raw_tags": [
        "Imprimerie"
      ]
    }
  ],
  "word": "D7"
}