A cancel element indicates that the old key signature should be cancelled before the new one appears. This will always happen when changing to C major or A minor and need not be specified then. The cancel value matches the fifths value of the cancelled key signature (e.g., a cancel of -2 will provide an explicit cancellation for changing from B flat major to F major). The optional location attribute indicates where the cancellation appears relative to the new key signature.
The
<cancel>element.A cancel element indicates that the old key signature should be cancelled before the new one appears. This will always happen when changing to C major or A minor and need not be specified then. The cancel value matches the fifths value of the cancelled key signature (e.g., a cancel of -2 will provide an explicit cancellation for changing from B flat major to F major). The optional location attribute indicates where the cancellation appears relative to the new key signature.
See
https://www.w3.org/2021/06/musicxml40/musicxml-reference/elements/cancel/