Thank you for your answer
Visitors can stay in Canada for up to six months, but we can request to extend their visitor visas up to another six months. My question is: If I can extend the visitor visa for my parents for another six months, during this time, can parents leave Canada, for example, for one year, and come back after that? With this assumption, they would stay just two months in the second six months' permission, and four months would be left.