What does “private topic” mean ?

I just got notification that member x quoted me in the post in “private topic”.
As i never got anything like this before i want to know what “private topic” is and how one can start a private topic.

Thanks

Disclaimer as I might be slightly OT with this answer.

Most of the time I have seen this is where a user was culled for some reason and PM's were also culled in the process.

Open to other opinions too.

Bob.

So kinda forum bug?

Not so much a BUG as a bad message description.