Entities can be given identifiers by other organisations when they suscribe to their service. This identifier can be seen as a contact, as it will be used to reference the entity in all further exchanges. As the number of suscription can be unlimited, creating a new property for each kind of identifier becomes heavy, but customizing contacts using getIdTranslationDict is easy.
As a real life use case, in UK, a company which is allowed to instruct direct debits to their customer bank accounts is given a Service User Number, which should be transmitted with all direct debit instructions.