In a multi-module Maven project, what could make the parent project not to be a part of the reactor?

[Behrang Saeedzadeh]

I was just reading the source code for the [Versions Maven Plugin](
http://mojo.codehaus.org/versions-maven-plugin/), and noticed this snippet
of code:

       if ( reactorProjects.contains( getProject().getParent() ) )
       {
           getLog().info( "Project's parent is part of the reactor" );
           return;
       }

When would the reactor not contain the parent module/project?

Best regards,
Behrang
http://www.behrang.org