The Ventra *CARD* itself is a Mastercard with NFC.

It can be used to access two different subaccounts:
A Ventra Transit subaccount that is used to pay for bus and train rides. This subaccount may contain cash (known as "transit value") or transit passes.

An optional prepaid debit Mastercard subaccount. This option is turned off by default. Even thought every Ventra Card is capable of acting as a prepaid debit Mastercard, you have to take affirmative steps in order to activate this capability.

The two subaccounts are kept separately. You may use the debit subaccount in order to load the transit subaccount, but not vice versa. The debit subaccount has fees associated with it if you choose to activate it. Many people assumed those fees would apply even if you didn't activate the debit subaccount, which caused much controversy (and still continues to cause some).