Check out the official guide:
groupId uniquely identifies your project in all projects, so we need to enforce the naming scheme. It must follow the rule package name, which means it should be at least a domain name that you and you can create as many subgroups as you want. look at more information on package names.
eg. org.apache.maven, org.apache.commons
A good way to determine the granularity of groupId is to use the project structure. That is, if the current project is a multiple module, it must add a new parent group identifier identifier.
eg. org.apache.maven, org.apache.maven.plugins, org.apache.maven.reporting
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