More or less correct and effectively so..
The fact that this works for the webapp poms in the SDK is kind because of the way maven resolve parent references...
Maven looks for the parent pom first in the reactor of currently building projects, then in this location
on the filesystem, then the local repository, and lastly in the remote repo. relativePath allows you to
select a different location, for example when your structure is flat, or deeper without an intermediate
parent pom. However, the group ID, artifact ID and version are still required, and must match the file
in the location given or it will revert to the repository for the POM.
In the SDK case you will see that the backend webapp module actually has a parent declaration that matches (as in equals) the ../pom.xml, which in turn has a parent declaration that matches it's own ../pom.xml being the SDK root pom. This is not true for your WCB and therefore it fails.
So, basically this kind of works by accident and you can and should not use it outside your own project. In general I prefer to stick a project pom between the webmanager-wcbs pom and my own WCB poms so I can control the webmanager version an possibly other global settings in one place.
Regards,
Bram