I've been trying to understand this myself for the last few days. I think this is how it works:
"build->default" means that if I ask for all of the "build" dependencies of my jar, then Ivy pulls the "default" version of that dependency from the repository. If instead you say "build->build" or equivalently "build" then Ivy looks for a jar tagged "build" in the repository to satisfy that dependency. I think the main purpose of having multiple tagged artifacts in the repo are to have a "default", "source", "javadoc" and maybe "debug" version of your code.
I make heavy use of "provided->default" to specify jars that I need to compile against, but that I don't need to add to my plugin installer because I know that jar will already be present at runtime because it's provided by a different installer.