Aides-de-camp carry all your orders no matter what the setting is for Fog of War. The unit info card for the commanders shows the number of AdCs currently available, it's under the quill pen icon. When you send an order, that number will decrease by one until that AdC returns. If you've sent all your AdCs out with orders, you cannot issue another order until one or more return.
The second option (aides) will send an AdC to each corps commander to get updated information from him. Useful for Fog of War. The white number in the lower left of the unit info card is the time that unit has been out of contact with the CinC. With some Fog of War settings, a higher number there indicates that your information on that unit's location may be out of date.
I don't know if an AdC is required to give an order to an individual regiment, or if an AdC is sent to each detached regiment to get a status update.
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