What is considered someone else's post?

Now on the main page hangs a message of the internal voice as follows:

Published someone else's post - lost account

If I understand correctly, it comes from the administration.
The choice of such a format instead of a normal post is strange in itself, but this is not the point.

My question is what is considered “someone else's post”?

I see three options:

1. Kopipast. The person copied the entry from a newspaper or someone else's blog and posted on his own behalf.
2. Just posted will announce. The man published a post written by a friend-acquaintance by mutual agreement with him.
3. Instead of a sandbox. The man published a post written by his friend, a friend, asking for an invite.

For which of these posts can you lose your account?