While people aren't required to have the DNI on hand at all times, they are required to identify themselves and give it to the police officer if having it on themselves. If a DNI is required but can not be given, another document can be given in substitution for the document, even if that document is not officially a way to identify themselves. For example, under Spanish law, a driver's license isn't a document to identify a person, but an authority can choose to admit it.