A dynamic field is a field whose name, or label, is determined by an expression wrapped in parentheses or through string interpolation: (a + b) or "\(a + b)".

A dynamic field’s identifier is not available in the scope in which the field is defined.
Referencing dynamic fields needs to be done using selectors, index expressions, or aliases.

a:       "foo"
b:       "bar"
(a + b): "foobar"

s: X={
	"\(a)_and_\(b)": "foobar"

	// Valid references using a selector and
	// an index expression.
	FooAndBar: s.foo_and_bar
	FooAndBar: X["foo_and_bar"]

	// Invalid reference because the
	// indentifer is not in scope.
	//FooAndBar: foo_and_bar

// Valid reference using an index expression.
FooAndBar: s["foo_and_bar"]
$ cue eval file.cue
a: "foo"
b: "bar"
s: {
    foo_and_bar: "foobar"
    FooAndBar:   "foobar"
foobar:    "foobar"
FooAndBar: "foobar"