The issue confuses new techies I mentor. In the image above the before tag is stating that the class should be executed before the File['copy-init-file'].
The reason this can confuse you for the first time, is where you compare this to the require tag. This states that Exec['unarchive-source] is required to run 'before' this class.
Some of this can be implied by the layout of the code, however it should be stressed that the layout order of the code only effects the catalogue compilation, not the execution.
If you would like me to explain in more detail, we can discuss below.
Links: Slides