The visa stamps in passports are only used for admission to the U.S. in a specific status. Upon entry into the U.S. visa stamps are not the controlling document for legal status in the U.S. A valid, unexpired visa is not required for maintenance of legal status. If you will be traveling abroad during your program and plan to return to the US to continue your program and your visa stamp has expired visa it must be renewed prior to returning to U.S. Do not mistake the validity of your visa for the duration of approved stay in the U.S.
The visa stamp also indicates whether it is a single (S or 1) or multiple (M) entry visa. A multiple entry visa allows the bearer to use the visa enter the US to continue their current J-1 program as many times as they want or need to do so as long as the visa and the DS-2019 form are both still valid.
Two Year Home Residency Requirement
The visa will also indicate whether the bearer is subject to section 212(e) of the Code of Federal Regulations. Also known as the two-year home residency rule.