In the marks section in vega, the from pragma points to the dataset that should be used: for every datapoint in the dataset, a single mark is created. If no from is provided, vega defaults to creating a single mark. Let’s for example add a red square and a bit of text to the plot above:

{
  "$schema": "https://vega.github.io/schema/vega/v5.json",
  "width": 400,
  "height": 200,
  "padding": 5,

  "data": [
    {
      "name": "table",
      "values": [
        {"x": 15, "y": 8},
        {"x": 72, "y": 25},
        {"x": 35, "y": 44},
        {"x": 44, "y": 29}
      ]
    }
  ],

  "scales": [
    {
      "name": "xscale",
      "domain": {"data": "table", "field": "x"},
      "range": "width"
    },
    {
      "name": "yscale",
      "domain": {"data": "table", "field": "y"},
      "range": "height"
    }
  ],

  "marks": [
    {
      "type": "symbol",
      "from": {"data":"table"},
      "encode": {
        "enter": {
          "x": {"scale": "xscale", "field": "x"},
          "y": {"scale": "yscale", "field": "y"}
        }
      }
    },
    {
      "type": "rect",
      "encode": {
        "enter": {
          "x": {"value": 50},
          "y": {"value": 50},
          "width": {"value": 30},
          "height": {"value": 30},
          "fill": {"value": "red"}
        }
      }
    },
    {
      "type": "text",
      "encode": {
        "enter": {
          "text": {"value": "This is some text"},
          "x": {"value": 100},
          "y": {"value": 50}
        }
      }
    }
  ]
}

Notice that there is no from defined for the square and the text.