The difference between AIS B (sensory incomplete) and AIS C (motor incomplete) is the preservation of motor function below the neurological level of injury.
The difference between AIS C and AIS D is how much motor function is spared.
You can find more details about the specific classifications on the second page of the ISNCSCI handouts in the middle column.
In this case, the patient has voluntary anal contraction, so we know they have a motor incomplete injury and their level will be C, D, or E. The NLI is C6. Below C6, only 3 out of 16 testable muscles bilaterally have MMT scores of 3 or greater (right elbow extensors and finger flexors, left elbow extensors). This is less than half, so the injury is classified as AIS C.