"separate cover" meaning in English

{
  "head_templates": [
    {
      "args": {
        "1": "-"
      },
      "expansion": "separate cover (uncountable)",
      "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": "other",
          "name": "Pages with 1 entry",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Pages with entries",
          "parents": [],
          "source": "w"
        }
      ],
      "examples": [
        {
          "ref": "1918, Bridge Men's Magazine, page 253:",
          "text": "[I am] sending under separate cover some pictures of the launching of the ship[…]",
          "type": "quote"
        }
      ],
      "glosses": [
        "Correspondence sent separately from other items."
      ],
      "id": "en-separate_cover-en-noun-da~NPtvP",
      "links": [
        [
          "Correspondence",
          "correspondence"
        ]
      ],
      "related": [
        {
          "word": "septel"
        }
      ],
      "tags": [
        "uncountable"
      ]
    }
  ],
  "word": "separate cover"
}

{
  "head_templates": [
    {
      "args": {
        "1": "-"
      },
      "expansion": "separate cover (uncountable)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "related": [
    {
      "word": "septel"
    }
  ],
  "senses": [
    {
      "categories": [
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "English terms with quotations",
        "English uncountable nouns",
        "Pages with 1 entry",
        "Pages with entries"
      ],
      "examples": [
        {
          "ref": "1918, Bridge Men's Magazine, page 253:",
          "text": "[I am] sending under separate cover some pictures of the launching of the ship[…]",
          "type": "quote"
        }
      ],
      "glosses": [
        "Correspondence sent separately from other items."
      ],
      "links": [
        [
          "Correspondence",
          "correspondence"
        ]
      ],
      "tags": [
        "uncountable"
      ]
    }
  ],
  "word": "separate cover"
}