Question: When do the Detachments come in? When do they move?
The Scenario specifies how many Detachments for each side. It doesn’t say when or where they arrive. Why? Because you decide that! You assign them to specific Commands for the battle, before the game. They enter or setup with that Command. They move with that Command.
The point of the Detachments, is to create more uncertainty and fog-of-war. That’s why the OB doesn’t specify which one they start with. It would defeat the purpose. How many more blocks do you have to fight with? Is that a real division or baggage train sitting back there? Or just a Detachment? You can’t tell.
If you put them out front, Detachments can be good for screening and delaying the enemy sometimes but they will likely get killed quickly. I find them more valuable in a reserve position for deception. In fact, I often like to keep them on the Reserve Card. I can throw them out to back up a defense when things get desperate. Sometimes an attacker will call off a fight and fall back before he sees that the scary looking support I have is only a Detachment.
If he only knew!