There are two types of Canadian visitor visas: single entry visas and multiple entry visas. A single entry visa allows foreign nationals to enter Canada for a one-time-only. A multiple entry visa allows holders to enter and leave Canada as often as they want as long as the visa is valid. You don’t need to choose which kind to apply for, applicants are automatically considered for multiple entry visas and are only issued single-entry visas under unique circumstances.
Multiple entry visitor visas permit the holder to travel to Canada for six months at a time as many times as they want, as long as the visa remains valid. They can be valid for up to 10 years, but the exact validity period is at the discretion of the visa officer issuing it. If you have some other status document, such as a study permit or a work permit, and are not otherwise visa-exempt, you will automatically be issued a visitor visa allowing you to enter Canada to receive your permit. Usually, this is a multiple entry visa. If you choose to temporarily leave Canada during the course of your studies or temporary work, you will not need to apply for a new visitor visa to re-enter Canada as long as your permit and visa are both still valid.