How does sbt 0.13 choose the configuration scope to use?

Question

sbt "scopes" its tasks and settings. Documentation talks about the "scope axes" One of them is the "configuration axis" which seems to be magically selected for the evaluation of a task. Evaluate "compile" and the configuration scope is "Compile". Evaluate "test" and the configuration scope is "Test".

I cannot find anything in the documentation on how this selection is done. The most specific description on this topic I found was in James Roper's "all that jazz" blog. It says:

When you don’t specify a scope, sbt will choose a default scope, in this case it has chosen the "compile" scope. The logic in how it makes that decision we won’t cover here.

Pretty terse, actually. So, my questions:

• Is there anywhere else a coverage on that specific "logic"?

• How does sbt choose the "configuration" scope it applies to the task/settings evaluation?

• Which "configuration" scope values exist? Is this a fixed list? Where do the existing configuration scope values come from? How (and why) to create new ones?

• Is there any "general idea" behind this whole scoping thing?

I feel that this is a key to understand sbt in general but I must admin that I currently have almost no clue on how sbt acts here and why it acts like this. Can anyone enlighten me?