Silas Arentsen Hi Silas, both of these codes are called BIC (Business Identifier Code). Calling them SWIFT codes is just an unofficial term, because of their origins in payment transfers through the SWIFT network. Nowadays these codes are used in many different fields and purposes.
What is true, though, that both BUNQNL2A and BUNQNL2AXXX are equally valid, as a BIC can have 8 or 11 digits. In case of 11 digits, the last 3 digits are for identifying a certain branch or department of the business in question. When no specific branch shall be specified, you can either use the 8-digit version or you can put "XXX". Both of these codes have therefore the same meaning.