By now I think some people are getting the hang of the new APFS Mac file system. It really is quite intelligent once you get used to it. It really is easy to create several Volumes within a Container structure for testing purposes or whatever else you wish to do. If you have created Volumes in a Container, you may wish to Delete one or more of them at some point. This article explains how to do that with the caveat of making a good backup of the Data first.