I take NA as meaning this item does not apply to this person at this time. (exempt, as you say - it does not pertain to his job classification) I would say that "Not Observed" is the correct response when you as an evaluator have not had the opportunity to observe the person in the performance of this particular task - or have not seen the results of his work in this particular area. It doesn't mean he has neglected to perform the task.
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