That is exactly it. Thank you!
My example:
add_filter('acf/validate_value/name=event_category', 'validate_empty_fields', 10, 4);
function validate_empty_fields( $valid, $value, $field, $input ){
// bail early if value is already invalid
if( !$valid ) {return $valid; }
$is_event_public = $_POST['acf']['field_55420ab8d2eb2']; // This field is a checkbox
if($is_event_public){
if(!$value){
$valid = __('This field is required for public events');
}
}
return $valid;
}