What's a specific browsing context?

So, window.open creates an auxiliary browsing context; per HTML:

It is possible to create new browsing contexts that are related to a top-level browsing context without being nested through an element. Such browsing contexts are called auxiliary browsing contexts. Auxiliary browsing contexts are always top-level browsing contexts.

WebDriver talks about command contexts as follows:

Many WebDriver commands happen in the context of either the current browsing context or current top-level browsing context. The current top-level browsing context is represented in the protocol by its associated window handle. A top-level browsing context is selected using the Switch To Window command. Once this is done, a specific browsing context can be selected using the Switch to Frame command.

Given this, what do I use to switch to an auxiliary browsing context?

Complicating this is the question of what's a "specific browsing context"? It's not cross-referenced, and it doesn't seem to be defined anywhere. The definition of it seems to imply that it should be "child browsing context" (and Switch to Parent Frame switches to the "parent browsing context")?

As such, I assume I need to use Switch to Window command to change to an auxiliary context. Is the only way to get the handler for that return window.open('http://example.com') and rely upon the JSON serialisation of the WindowProxy?

Start with the HTML and WebDriver quotations in the issue, then inspect the specification's definitions of browsing contexts, window handles, and frame switching. Resolve whether auxiliary contexts are selected with Switch to Window and document the meaning of “specific browsing context” and how its handler is obtained.