In most of the multi-module Maven projects, there are many things that are shared across multiple modules. Dependency versions, plugins versions, properties, and repositories are only some of them. It is a common as well as a best practice to create a separate module called parent, and in its POM file, define everything in common. The packaging type of this POM file is pom. The artifact generated by the pom packaging type is itself a POM file.

The following are a few examples:

Not all the projects follow this approach. Some just keep the parent POM file under the root directory (not under the parent module). The following are a couple of examples:

Both approaches deliver the same results. However, the first one is much preferred. With the first approach, the parent POM file only defines the shared resources across different Maven modules in the project, while there is another POM file at the root of the project, which defines all the modules to be included in the project build. With the second approach, you define all the shared resources as well as all the modules to be included in the project build in the same POM file, which is under the project's root directory. The first approach is better than the second one, based on the separation of concerns principle.