If you are using the PayPal site or checking out with PayPal on a merchant site, your purchase can be funded by your balance, credit card or bank transfer.

If you have a PayPal Debit Card (a Mastercard that allows you to "charge" things against your PayPal balance), it will no longer have a back up funding source. You can only used the debit card for as much as you have in your PayPal account. I think that's pretty much typical with any kind of bank account debit card unless you have a line of credit.