Otredou Actually there is no difference:
Travel Card accounts don't have a public IBAN, but the Travel Card has an internal account reference used by bunq (or for that matter any credit card issuer) to keep track of the spendings and deposits.
For Premium subscribers that account is not an internal account reference, but one of their public IBANs, whichever they connect their card to.
So Premium subscribers have to top up their card as well, but in another fashion, being a balance on a regular account.
Both systems are in practice similar to any pre paid credit card scheme out there.
As for the difference between the debit Mastercard and the Travel Card: the difference is the way the payment is processed. Credit card payments carry much more information than your regular transfer (such as a reservation marker or a clearance marker et cetera).
This is all done in the background and customers won't see the difference at a first glance.