In order to identify itself, the Company can add the Registration No. in Company information, but in order to identify the Customer or Vendor there is no such opportunity in D365 Business Central standard. It should be taken into consideration that not all local companies have VAT Registration numbers, moreover, you cannot fully rely on the company name, because the name of the company also may be changed in the Chamber of Commerce/Company House/Company Registrar etc. throughout the life of the company. Therefore Company Registration No. is a unique identifier by which we can check whether the company is in operation (majority of EU countries have an open registrar, so that the business could check their customers and vendors. If there is no unique identifier, then a risk of mistakes is very high, that we can have several customers with an opening balance, that are actually one company.
Registration No. is a field that should be available for Customer, Vendor, Contact cards.