Switch To Frame: clarify language when selecting frame by index

The section switch to frame in the specs is a little unclear in regards of what "ID" is supposed to be.
According to the specs:

If id is not null, a Number object, or an Object that represents a web element, return error with error code no such frame.

While id is null and id represents a web element are clear, id is a Number object leaves me with the question: "which number"? The link to supported properties index points to the IDL specs' cover of supported property index.

So... where do I get this number from, exactly, if I wanted to use to switch frame?

Please remember that I am not talking about programming languages API -- I am talking about the usage of the API without support library. I am specifying this because the IDL link seems to be all about Java, whereas my question is on the use of the Webdriver endpoints.

Start with the Switch To Frame section in the WebDriver specification and follow its supported property index link to the Web IDL definition. Clarify what the numeric frame identifier represents and how a direct WebDriver endpoint caller obtains it; the section should answer the issue's question without relying on a programming-language API.