The Site Mirror is a copy of the site at another address. The Website is seen in this case in the Internet under different names.
Typically only one of the mirrors (the main one) is provided in search results. To indicate to Yandex which of the mirrors is main, one must specify a Host Directive in the file robots.txt:
User-Agent: *
Disallow:
Host: www.main-mirror.com
The Host Directive plays important role at the choice of the main mirror, but doesn't give any guarantees for a necessary site to be selected.
If the search engine finds site mirrors and at the same time there isn't on any of the mirrors the file robots.txt with instructions for the robot, the search engine will glue them together to its discretion.