"3B" meaning in Conventions internationales

{
  "categories": [
    {
      "kind": "other",
      "name": "Mots en conventions internationales issus d’un mot en anglais",
      "parents": [],
      "source": "w"
    },
    {
      "kind": "other",
      "name": "Symboles en conventions internationales",
      "parents": [],
      "source": "w"
    },
    {
      "kind": "other",
      "name": "Conventions internationales",
      "orig": "conventions internationales",
      "parents": [],
      "source": "w"
    }
  ],
  "etymology_texts": [
    "De l’anglais black (« noir »)."
  ],
  "lang": "Conventions internationales",
  "lang_code": "conv",
  "pos": "symbol",
  "pos_title": "Symbole",
  "senses": [
    {
      "glosses": [
        "Dureté des mines tendre, entre 2B et 4B."
      ],
      "id": "fr-3B-conv-symbol--85-MZbx"
    }
  ],
  "word": "3B"
}

{
  "categories": [
    "Mots en conventions internationales issus d’un mot en anglais",
    "Symboles en conventions internationales",
    "conventions internationales"
  ],
  "etymology_texts": [
    "De l’anglais black (« noir »)."
  ],
  "lang": "Conventions internationales",
  "lang_code": "conv",
  "pos": "symbol",
  "pos_title": "Symbole",
  "senses": [
    {
      "glosses": [
        "Dureté des mines tendre, entre 2B et 4B."
      ]
    }
  ],
  "word": "3B"
}