The Digital Output Card supports a separate selection (arming) circuitry for each output channel in the card. This scheme is superior to the schemes which rely on a master relay common to a group of output channels and external sensing of the armed channels using digital inputs.
A common master relay scheme renders the whole group of output channels unusable in case a single point of failure occurs in the path of triggering the master relay. In contrast, in the on-board channel arming scheme, any failure in the channel arming/triggering will result in only a single DO channel becoming faulty.
Also, external sensing of armed channel using digital inputs requires extra wiring and consumes extra digital inputs. The on-board arming circuitry requires no extra wiring and requires no additional digital inputs.