First of all, it is about deposits to accounts
[sum of family's last 3 years income] + 10,000 rubles > [sum of family's deposits to all accounts for the year of control].
Then, the law states that prosecutor's office should requests the explanations about the source of deposits from an official. Only if the explanations are not provided, the funds could be seized.
http://publication.pravo.gov.ru/Document/View/00012022030600... Article 8.2.1