"datum surface" meaning in English

{
  "forms": [
    {
      "form": "datum surfaces",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "datum surface (plural datum surfaces)",
      "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": "Engineering",
          "orig": "en:Engineering",
          "parents": [
            "Applied sciences",
            "Technology",
            "Sciences",
            "All topics",
            "Fundamental"
          ],
          "source": "w"
        }
      ],
      "glosses": [
        "A surface on a workpiece that establishes a datum."
      ],
      "id": "en-datum_surface-en-noun-1a3xGct9",
      "links": [
        [
          "engineering",
          "engineering#Noun"
        ],
        [
          "surface",
          "surface"
        ],
        [
          "workpiece",
          "workpiece"
        ],
        [
          "datum",
          "datum"
        ]
      ],
      "raw_glosses": [
        "(engineering) A surface on a workpiece that establishes a datum."
      ],
      "topics": [
        "engineering",
        "natural-sciences",
        "physical-sciences"
      ]
    }
  ],
  "word": "datum surface"
}

{
  "forms": [
    {
      "form": "datum surfaces",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "datum surface (plural datum surfaces)",
      "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",
        "en:Engineering"
      ],
      "glosses": [
        "A surface on a workpiece that establishes a datum."
      ],
      "links": [
        [
          "engineering",
          "engineering#Noun"
        ],
        [
          "surface",
          "surface"
        ],
        [
          "workpiece",
          "workpiece"
        ],
        [
          "datum",
          "datum"
        ]
      ],
      "raw_glosses": [
        "(engineering) A surface on a workpiece that establishes a datum."
      ],
      "topics": [
        "engineering",
        "natural-sciences",
        "physical-sciences"
      ]
    }
  ],
  "word": "datum surface"
}