CUE defines the following type hierarchy:

CUE defines the value top (or any), written “_”, such that all types are an instance of top, and the value bottom (or error), written “_|_”, which is an instance of all types.

We can mix the terms types and values interchangeably because CUE doesn’t distinguish between types and values.
The term “type” merely refers to the kind of a value, which may or may not be a concrete instance.

In the following hypothetical example, point defines an arbitrary point, while xaxis and yaxis define any point on their respective axes. We say that point, xaxis, and yaxis are incomplete, as they don’t contain values that specify a specific point. Incomplete values cannot be included when exporting to formats that can’t represent them. JSON and YAML, for example, have no way to represent the type number.

point: {
	x: number
	y: number
}

xaxis: point
xaxis: y: 0

yaxis: point
yaxis: x: 0

origin: xaxis & yaxis

$ cue eval file.cue
point: {
    x: number
    y: number
}
xaxis: {
    x: number
    y: 0
}
yaxis: {
    x: 0
    y: number
}
origin: {
    x: 0
    y: 0
}

By contrast, origin is not incomplete, as it contains only concrete values. However, notice that we didn’t need to specify its values explicitly. CUE is able to infer from the constraints applied, placing origin on both the x-axis and y-axis, that its coordinates must be (x = 0, y = 0).