Answer
CommandRef lets you add a reference to an already-existing Command to a Command Group/Content collection, rather than defining a brand-new command inline — conceptually the same pattern as adding a Group element to a page, which is really just a reference pointing at a Group definition rather than the Group living directly on the page. Use it when a command you want to enable in a given Command Group already exists elsewhere. One important caveat: the command picker doesn't filter the list down to only commands that are actually valid for the current context — as the person configuring it, you're responsible for confirming the referenced command genuinely applies where you're adding it.