Signature Debit is used without a PIN but it is not PINless Debit. This is just one of the many perplexing realities of the payment world. You really have to leave your common sense at the door. It is true that you would not be using a physical signature to "sign" a PINless Debit transaction but you would be using your electronic "signature".
PINless Debit is a type of transaction where a consumer can pay a very limited type of bills such as a Utility bill, mortgage payments, etc. with the same card they otherwise would use at the ATM with a PIN or at the POS with a signature or a PIN or online as a signature debit. PINless Debit transactions are processed by the ATM networks such as Star, NYCE, Exchange, etc. The card may have a Visa or MasterCard logo on it but for these unique transaction types and with certain merchants who have been individually qualified by each ATM network, they would be processed as PINless debit. Interlink/PLUS and Maestro/Cirrus which are the Visa/MC PIN based POS/ATM networks do not support PINless Debit. These transactions are priced with a % fee but it usually caps out at around $.50.
Signature debit (used to be referred to as check cards) is where a debit card that also has a Visa or MC logo on it can be used for any retail or online purchase and when it shows up at the bank the funds are debited from the consumer’s checking (DDA) account. These transactions are priced like all Visa/MC transactions under Interchange albeit slightly less than credit card interchange but there isn’t a cap on the fee.
As an example, I have a Wells Fargo card that doubles as my ATM card and as my signature debit card. Depending on where I use it and which button I press on the POS terminal the transaction travels over a different processing network and the merchant would pay a different fee. If I were to use it at my mortgage company online, (and they had set up all the right back end systems and agreements), the transaction would run across the STAR ATM network rails. If I use it at a BofA ATM and enter my PIN, it likely would travel across the PLUS or STAR networks. If I use it at the POS and enter my PIN, it would travel across the Interlink network (owned and operated by Visa) and if I use it online to shop at AMAZON, it would travel across the Visa network.
Have I completely confused you? I have gone back and read this post over a few times and even I am confused. Oh well!