"free substitution" meaning in English

{
  "forms": [
    {
      "form": "free substitutions",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "~"
      },
      "expansion": "free substitution (countable and uncountable, plural free substitutions)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "topical",
          "langcode": "en",
          "name": "Sports",
          "orig": "en:Sports",
          "parents": [
            "Human activity",
            "Human behaviour",
            "Human",
            "All topics",
            "Fundamental"
          ],
          "source": "w"
        }
      ],
      "glosses": [
        "A rule in some sports that allows players to enter and leave the game for other players many times during the course of the game, and for coaches to bring in and take out players an unlimited number of times."
      ],
      "id": "en-free_substitution-en-noun-lHOvNTTA",
      "links": [
        [
          "sports",
          "sports"
        ]
      ],
      "raw_glosses": [
        "(sports) A rule in some sports that allows players to enter and leave the game for other players many times during the course of the game, and for coaches to bring in and take out players an unlimited number of times."
      ],
      "tags": [
        "countable",
        "uncountable"
      ],
      "topics": [
        "hobbies",
        "lifestyle",
        "sports"
      ]
    }
  ],
  "word": "free substitution"
}

{
  "forms": [
    {
      "form": "free substitutions",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "~"
      },
      "expansion": "free substitution (countable and uncountable, plural free substitutions)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        "English countable nouns",
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "English uncountable nouns",
        "en:Sports"
      ],
      "glosses": [
        "A rule in some sports that allows players to enter and leave the game for other players many times during the course of the game, and for coaches to bring in and take out players an unlimited number of times."
      ],
      "links": [
        [
          "sports",
          "sports"
        ]
      ],
      "raw_glosses": [
        "(sports) A rule in some sports that allows players to enter and leave the game for other players many times during the course of the game, and for coaches to bring in and take out players an unlimited number of times."
      ],
      "tags": [
        "countable",
        "uncountable"
      ],
      "topics": [
        "hobbies",
        "lifestyle",
        "sports"
      ]
    }
  ],
  "word": "free substitution"
}