What are modern POS systems sending to the banks and visa when you make a transaction?
Of course there would be the amount charged, but is the itemization from the receipt sent as well? For example, if I bought a $10.83 book, and 83 cents of that was tax, does VISA or my bank actually know that?
POS systems don't do the bank interface anymore ... usually it is a piece of middleware sits on the POS that is used to control the actual interface to the bank and the PED (swipe devices).
POS says I need payment to the value of amount ... middleware takes over and the POS get's back the card number encrypted (PCI) , auth code and cryptogram if chip (EMV) was used.
Loyalty programs are a different matter altogether , full itemization is sent to the third party if you are enrolled in a program.
The answer depends on the vendor and your bank. Beyond the basic vendor, amount and description, some vendors transmit additional details like tax breakdown, travel class and line-item data. I found a PDF from Wells Fargo that details some of the information that's transmitted.
On the bank side, I've seen the Level 2/3 information exposed to users of a purchasing cards (read: company credit cards). On the monthly statements we'd see line-item details for some vendors, like Best Buy. Travel purchases included flight segment details, cabin class, etc.
Like most things it depends. Different payment-gateways support different levels of integration. The highest fidelity ones support itemization. The more information you provide typically reduces the fee that's applied.
For more information google "Level 2/3 Credit Card Transactions"