A struct may be open or closed, and is open unless it has been closed.

Both open and closed structs can have any field defined as members.
Closed structs can only be unified with structs that have fields permitted by the closed struct.

A closed struct can be created using the close() builtin, but is more commonly created using a definition, as demonstrated on the next page. A struct created with close() can’t have additional fields added elsewhere.

A: close({
	a: int
})

B: A & {
	b: 42 // validation failure
}

$ cue eval -i structs.cue
A: {
    a: int
}
B: {
    a: int
    b: _|_ // B.b: field not allowed
}