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:
The Apache Axis2 project, available at http://svn.apache.org/repos/asf/axis/axis2/java/core/trunk/modules/parent/
The WSO2 Carbon project, available at https://svn.wso2.org/repos/wso2/carbon/platform/trunk/parent/
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:
The Apache Synapse project, available at http://svn.apache.org/repos/asf/synapse/trunk/java/pom.xml