If you don't place the code snippet on all the site's pages, Yandex.Metrica will interpret a user as appearing on the site and then disappearing — statistics will show distorted data.

Yandex.Metrica uses cookies to identify unique users. If cookies are cleared, the browser is reinstalled, or the operating system is reinstalled, the user will be counted as a new user.

The site user's ID assigned by Yandex.Metrica.

The site user's ID assigned by the site owner.

In Settings → Upload data in Yandex.Metrica.