Pub Battles is a command focused system, as opposed to a combat focused system. This doesn’t mean combat isn’t important. It means that a lot of the details of combat are left out. When the composition of a Corps is being constructed, effectiveness as a unit is prioritized over the details of specific unit composition.
When designing a Pub Battles scenario, one can first divide the individual Army’s total manpower by 4,000 to get a rough idea of how many blocks to include. This is just the starting point. Then one must consider cavalry and artillery, and how it acted in the battle. How many blocks, if any, give the right feel? Then there is efficiency. If the units had exceptional leadership and troops, it might be appropriate to include an elite block. If there were a significant portion of green or hastily raised troops, then some militia might be in order.
When all that is done, the OBs are then extensively play-tested. How do they feel? Can they duplicate historical behavior. Maybe a block is added here, or taken away there. Maybe an elite is added/subtracted, ditto with militia.
The very last thing that is done is the naming of the blocks. The unit ID is purely added for color. Without them, the game feels lifeless and generic. With them, the game feels more real, more fun.
When a corps is in combat and one of the blocks is eliminated, say Hood’s Texans at Antietam, it does not mean that the Texan division has been lost (though it might), What it really shows is that the effectiveness of the Corps has been reduced to the point that it is no longer accurately modeled with an elite block.
Of course, it is much more fun to simply think that the last of Jackson’s stalwarts has fallen! No harm, no foul.
This is very true with Baggage Trains. What does a Baggage Train represent? It may represent actual bags, hospitals, reserves, etc. All that is known is that if the enemy reaches that point, it’s game over. In actual terms it’s that point where a force is broken, either the troop’s, or the commander’s, will to fight is gone. Every decisive combat has had that point. The trouble in game design is that it must be quantified.
The Baggage Train rule is that mechanic. It takes a hazy uncertain point, a point that only those in the moment can sense, and models it in the game. It is literally vague, so that it can be figuratively exact. That is elegant design.