Profile: matthewL-7

// This is to enable the filtering at all.
add_filter( 'toolset_rest_run_exposure_filters', '__return_true' );
 
// exclude "book-additional-notes" custom field from "book" custom post type.
add_filter( 'toolset_rest_expose_field', function(
    $expose_field, $domain, $group_slug, $field_slug, $element_type, $element_id
) {
    if( 'posts' === $domain && 'book' == $element_type && 'book-additional-notes' == $field_slug ) {
        return false;
    }
 
    // Different case - do not alter the result.
    return $expose_field;
}, 10, 6 );