Any transaction, regardless of the funding source.... Keep in mind that when a PayPal transaction can happen, you can actually do one transaction from multiple funding sources. Maybe I have a $50 cash balance sitting in my PayPal account, and I see a pair of jeans for $100 that I want to buy. I may use the $50 from my cash balance and then use the rest as a bank transfer or a credit card payment.
As merchants, they don't need to understand or know how the transaction gets divided by funding type. As I mentioned earlier, PayPal is providing a flat rate regardless of the funding type and funding source.