What does a group with no group id means?

As part of my assignment I was asked to find a group called + and to write in brief what I think of it. This group in /etc/group is shown as +:x:: which means it has no Gid and No users. But what does that means? what does having no Gid does to a group. I wrote that it might be invalid but apparently that's a wrong answer. I couldn't find the answer in any documentation or tutorial.