PC Banking API

What so-called "PC Banking API" might you talking about? That's a pretty generic statement! Since you rule out Quicken being specific, you aren't talking about the Quicken connections I trust (like Web Connect, Direct Connect, etc....)

superset of OFX).

Yep - two different Andrew's! We're both "old timers" here; I can't remember who was first. So saying that, QFX is similar to OFX (Google "QFX OFX differences") and you'll see a lot of threads on the subject. QFX appears to me to be a 'proprietary' version (that's why there's a"Q" in the name) of OFX that (one of the quotes from a thread I found stated :

".qfx is NOT the same as .ofx and Quicken will not import .ofx files.

Did Quicken intentionally choose this language to mislead purchasers? That is a question I cannot answer. No, they chose this language to make it proprietary to Quicken...therefore banks and financial institutions have to pay Quicken to use that .qfx format. It's all about the revenue production."

But Andrew's answer pointing you to the OFX documentation is a good one.

Good luck!

Phrase one is technically true; phrase two is literally true.

QFX is not "proprietary". Intuit can not stop you from creating a file that meets QFX standards (and can be imported into Quicken). The "format" of a QFX file is determined by (and adheres to) the OFX specs, and the data in a QFX file is unencrypted: there is no way that QFX files can be "proprietary"; Intuit does not own, nor control the use of, either the format or the contents of a QFX file.

[Note: Money (and any other financial software applications) can read/import QFX files with no special processing required (assuming well written OFX import code). No one needs Intuit's permission to do that.]

The OFX specifications include the option for applications to include "tags" unique to the company/application. Intuit uses one such tag to contain an identifier that they issue to a financial institution that Intuit has a download agreement with. So QFX files are 100% OFX compliant, but contain a piece of data that is created by Intuit. That bit of data is no more unique to Intuit than your bank account number is with your bank; and you don't call an OFX file "proprietary" because it contains a piece of data (your bank account number) that is issued to you by some corporation.

You're right about the reason that QFX contains (slightly) different "data" than OFX: the purpose of the difference between QFX and OFX is to allow Intuit to (try to) prevent users from IMPORTING plain OFX files from financial insitutions that do not have a download agreement with Intuit. And that is so Intuit can charge for importing that data (which they could not do with QIF file data).