Building in optionality for PSD2

PSD2 and open banking... That's all fine but what services do we have to place under "open banking"???? I suppose that we all agree it's about making payments and moving value from one site to another. From one bank to another bank/ledger/system, .... And for many, API seems to be the magic word here, but what exactly do they mean by API? Application Programmable Interface ..... To program one rule per bank "to interface to"? Creating a few thousands of rules all to become translated in that API that has to serve payments to ING, JPMorgan, PayPal, AliBaBa, .... ? In best case to be limited to a few local rulings like SEPA in Eurozone, NPP for Aussies, and others still to be build for Japan, China, US, UK, .... Or don't we rather have to think about one to serve them all? Just like IP, HTTP(s), SMTP did and still do for communication, webtexting, email....? One simple protocol that every value transmitting service... being a bank, PayPal or whatever, talks and understands? Something like those guys under the wings of the W3C right now are developing? The one they called ILP or the InterLedger Protocol? The one that you can find here: www.interledger.org ?