Looks like they are intending to get a commission (become an officer). Once a commission is accepted it is essentially for life unless resigned or taken for cause. A lot of junior officers were called up after 911 even though they had fulfilled their service obligation (normally 8 years) because their commission was still valid.
Only a regular commission.
Most commissioning programs offer a reserve commission, which functions much like a term of enlistment. Once the commitment is complete, the officer receives a formal discharge. It's those with a regular commission who remain "on the books", so to speak, and must formally resign their commission to avoid any future commitments.