To be more specific.
Option 1: Real people who choose to play as couriers are given a letter at HQ. This letter has worded orders written by the highest ranking officer on the field. The courier then is given a waypoint on his map or on his hud, displaying where the regiment's last known location was. He has to go there either on foot or on horse, track down the regiment, and give the colonel the letter. The colonel reads the letter, and acts accordingly.
Option 2: Real people who choose to play as couriers are given a letter at HQ. The letter is merely just a representation, and really holds no words. The highest ranking officer at HQ selects that he wants to create a letter, he configures new attack/defend/movement/patrol/camp/skirmish waypoints for the regiment in the letter. The letter is given to the courier. The courier is given a waypoint on his hud, displaying where the regiment's last known location was. He has to go either on foot or on horse, track down the regiment, and give the colonel the letter. The colonel opens the letter given. By doing so, it automatically places the selected waypoint on every officer in that regiment's hud, so they all know their new orders, and act accordingly.
Option 3: Same as option one, except the courier is AI.
Option 4: Same as option two except the courier is AI.
Option 5: Same as option one, except there is no actual courier and the letter automatically appears to the colonel after the appropriate delay depending on that regiment's distance from HQ.
Option 6: Same as option two, except there is no actual courier and the orders automatically appear to the colonel after the appropriate delay depending on that regiment's distance from HQ.
So, to be clear:
Option 1,3, and 5 all feature the colonel opening a letter. He must interpret the letter and then act. These are the most realistic and historically accurate options.
Option 2,4, and 6 all feature the colonel opening a "letter". After he opens the letter, it simulates that there were actual words in the letter, however it just updates ordered waypoints on his hud and map. It does the same for every other officer in the regiment.
If there were any viable relay options I missed, suggest them below.