The "T" should indicate that item (ex: Sub Transmission Oil) needs to be tended to at that mileage/hour marker. Whether that be inspected or replaced.
A blank spot would indicate nothing needs to be done at that mileage/hours.
Edit: It appears that Honda doesn't used the older way of labeling the schedule, the "T" is just a placeholder fo the item at that mileage/hour mark. The service manual uses a hollow circle to mark them.