Most debit cards are issued as 'credit cards' by visa or mastercard etc. This means you can use them like a debit card where you put in your PIN and money is transferred immediately from your bank account. This use is dangerous as password swipers or key stroke monitors can steal the PIN and use your data with immediate access to funds and little way to get it back.
However you can use the same debit card and use it like a credit card. Meaning you hit the credit button instead of debit and you DON'T put in the PIN. This money still comes out of your account but your PIN is safe and you have the full protection from unauthorized use or charges on that card similar to a standard credit card. Especially if you go to a hotel where they put a charge held on your card and you use a debit card you need to be sure it is used as a 'credit' transaction and not a debit with a PIN.
Confusing for sure, the whole economy is tied up in electronic and computerized transactions ripe for fraud and crime and it is very tough to protect anything or anyone much less change back to an all cash society.
I agree that those requiring a SSN should be given a phony one as it should not be used for identification purposes. One of my banks REQUIRES your SSN as your 'username' for logging in to online banking. How backwards is that?
