"side letter" meaning in English

{
  "forms": [
    {
      "form": "side letters",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "side letter (plural side letters)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Pages with 1 entry",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Pages with entries",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "langcode": "en",
          "name": "Law",
          "orig": "en:Law",
          "parents": [],
          "source": "w"
        }
      ],
      "glosses": [
        "A collective bargaining agreement that is not part of the underlying or primary collective bargaining agreement used by the parties to the contract."
      ],
      "id": "en-side_letter-en-noun-D-nvSF9S",
      "links": [
        [
          "law",
          "law#English"
        ]
      ],
      "raw_glosses": [
        "(law) A collective bargaining agreement that is not part of the underlying or primary collective bargaining agreement used by the parties to the contract."
      ],
      "synonyms": [
        {
          "word": "side agreement"
        }
      ],
      "topics": [
        "law"
      ]
    }
  ],
  "word": "side letter"
}

{
  "forms": [
    {
      "form": "side letters",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "side letter (plural side letters)",
      "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",
        "Pages with 1 entry",
        "Pages with entries",
        "en:Law"
      ],
      "glosses": [
        "A collective bargaining agreement that is not part of the underlying or primary collective bargaining agreement used by the parties to the contract."
      ],
      "links": [
        [
          "law",
          "law#English"
        ]
      ],
      "raw_glosses": [
        "(law) A collective bargaining agreement that is not part of the underlying or primary collective bargaining agreement used by the parties to the contract."
      ],
      "topics": [
        "law"
      ]
    }
  ],
  "synonyms": [
    {
      "word": "side agreement"
    }
  ],
  "word": "side letter"
}