This could use some explanatory docs too.  The other two items here are related to Require plugins -- things you use via a "!" prefix expression.  That's not true of "mode", so it's potentially confusing.  Am I correct that this is just creating a "path variable" of sorts that extensions can use to get at CM modules that we ship with but don't load by default?  And that this isn't merely a convenient shorthand, but the only way that such extensions could point to these modules reliably?