"H-2" meaning in Translingual

{
  "head_templates": [
    {
      "args": {
        "1": "mul",
        "2": "noun"
      },
      "expansion": "H-2",
      "name": "head"
    }
  ],
  "lang": "Translingual",
  "lang_code": "mul",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "Pages with 1 entry",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Pages with entries",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Translingual entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Translingual links with redundant target parameters",
          "parents": [
            "Links with redundant target parameters",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Translingual links with redundant wikilinks",
          "parents": [
            "Links with redundant wikilinks",
            "Entry maintenance"
          ],
          "source": "w"
        }
      ],
      "coordinate_terms": [
        {
          "word": "D"
        },
        {
          "alt": "protium",
          "word": "H"
        },
        {
          "word": "T"
        }
      ],
      "glosses": [
        "deuterium (“hydrogen-2”)"
      ],
      "id": "en-H-2-mul-noun-xqEd7f1m",
      "links": [
        [
          "deuterium",
          "deuterium#English"
        ]
      ],
      "related": [
        {
          "word": "H-1"
        },
        {
          "word": "H-3"
        },
        {
          "word": "H2"
        },
        {
          "word": "HII"
        }
      ],
      "synonyms": [
        {
          "word": "D"
        }
      ]
    }
  ],
  "word": "H-2"
}

{
  "coordinate_terms": [
    {
      "word": "D"
    },
    {
      "alt": "protium",
      "word": "H"
    },
    {
      "word": "T"
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "mul",
        "2": "noun"
      },
      "expansion": "H-2",
      "name": "head"
    }
  ],
  "lang": "Translingual",
  "lang_code": "mul",
  "pos": "noun",
  "related": [
    {
      "word": "H-1"
    },
    {
      "word": "H-3"
    },
    {
      "word": "H2"
    },
    {
      "word": "HII"
    }
  ],
  "senses": [
    {
      "categories": [
        "Pages with 1 entry",
        "Pages with entries",
        "Translingual entries with incorrect language header",
        "Translingual lemmas",
        "Translingual links with redundant target parameters",
        "Translingual links with redundant wikilinks",
        "Translingual nouns",
        "Translingual terms spelled with numbers"
      ],
      "glosses": [
        "deuterium (“hydrogen-2”)"
      ],
      "links": [
        [
          "deuterium",
          "deuterium#English"
        ]
      ],
      "synonyms": [
        {
          "word": "D"
        }
      ]
    }
  ],
  "word": "H-2"
}