Top, written _, represents any CUE value.
It can be thought of as a placeholder, waiting to be given a more specific value elsewhere.

A field with a value of top can’t be exported because data formats such as JSON and YAML don’t have any way to represent non-concrete values.

Constraining a regular field with top brings that field into existence, whilst allowing its value to be constrained and defined elsewhere.

a: _
b: _
c: _
d: _
e: _
f: _

a: 1
b: 2.2
c: "three"
d: {four: "five"}
e: [6, 7.7, "eight"]
f: true

$ cue export fields.cue values.cue
{
    "a": 1,
    "b": 2.2,
    "c": "three",
    "d": {
        "four": "five"
    },
    "e": [
        6,
        7.7,
        "eight"
    ],
    "f": true
}