"221" 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 du chemin de fer",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Wiktionnaire:Exemples manquants en conventions internationales",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Conventions internationales de France",
          "orig": "conventions internationales de France",
          "parents": [],
          "source": "w"
        }
      ],
      "glosses": [
        "Disposition d’essieux de locomotive à vapeur comportant un bogie avec deux essieux porteurs suivi de deux essieux moteurs puis un essieu porteur arrière."
      ],
      "id": "fr-221-conv-symbol-g5mOU-Dc",
      "raw_tags": [
        "Chemin de fer",
        "France"
      ]
    }
  ],
  "synonyms": [
    {
      "word": "4-4-2"
    },
    {
      "word": "2B1"
    }
  ],
  "word": "221"
}

{
  "categories": [
    "Symboles en conventions internationales",
    "conventions internationales"
  ],
  "lang": "Conventions internationales",
  "lang_code": "conv",
  "pos": "symbol",
  "pos_title": "Symbole",
  "senses": [
    {
      "categories": [
        "Lexique en conventions internationales du chemin de fer",
        "Wiktionnaire:Exemples manquants en conventions internationales",
        "conventions internationales de France"
      ],
      "glosses": [
        "Disposition d’essieux de locomotive à vapeur comportant un bogie avec deux essieux porteurs suivi de deux essieux moteurs puis un essieu porteur arrière."
      ],
      "raw_tags": [
        "Chemin de fer",
        "France"
      ]
    }
  ],
  "synonyms": [
    {
      "word": "4-4-2"
    },
    {
      "word": "2B1"
    }
  ],
  "word": "221"
}