Here is what he wrote:
To clear it somewhat up. The Icon in the top is always shown as attack if one unit is in attack stance or assault. It does not effect the whole battle though. Units in the region on defense and entrenched will use their defensive fire stats and the leaders defense value. A unit in the back will join the Battle. As it MTSGs it will never have entrenchments and act is if it has 0 entrenchment. Units in attack will "attack" and use their leaders attack value. That means they will also according to their stance take on the penalties and bonuses.
So lets say German General A is being attacked in wooded Hills. He will begin the engagement and start firing on the attack which as the attack receives the attacker penalties. Behind the Frontline is German General B he is on attack and will thus "counterattack" and receive the offensive penalty but take advantage of his leaders high offensive skill thus maybe even negate the penalties. Next to G.General B is G.General C who is in defensive stance. He will join the battle and use the terrain in his defense but lack any and all entrenchments.
Thus if the terrain is clear it might actually makes sense to put a good and high offensive General for Example Hutier on attack to have him MTSG using his high strat/offensive skill to deploy both better and more troops.
Here is my version, based on going over his line by line. So credit to him:
The Posture Icon on the top of the Battle Report is always shown as Offensive Posture if one stack is in Offensive Posture. But this posture does not effect all units for the entire battle.
A unit in a military stack with a Defensive Posture and entrenched will use their defensive fire stats and the Commanders Defense Value, a unit in a stack with an Offensive Posture will use their offensive fire stats and the Commanders Offense Value. When on any posture, the unit gains all the bonuses and penalties specific to the terrain and weather.
A military stack on defensive or passive posture moving into or already within a territory with under 5% friendly Military Control (unless composed entirely of cavalry, Irregulars or support units) wil revert to offensive posture and automatically engage enemy units in that territory.
A unit will March To Sound of Guns (MTSG), if it is not set to evade, regardless of posture, and join the combat. When a unit MTSG's it will combat without the bonus of entrenchments.
Let us say that Commander von Below is within Wooded Hills. His military stack is on Offensive Posture in an attempt to catch French General Dubail's troops who have 5% Military Control and set to Defensive Posture.
Belows stack rolls a Detection Roll and Detects the enemy, his stack initiates the engagement and begins the attack. Within Wooded Hills in the Snow the German Stack receives the Attacker Penalties from weather and Terrain.
In a territory adjacent to the Wooded Hills is German General Falkenhayn, his Military Stack is also set to Offensive Posture and when his stack MTSG's they will receive the attacker penalty. But due to Falkenhayn's high offensive skill many of the attacker penalties may be negated.
In a second territory adjacent to the Wooded Hills is German General Heeringen on Defensive Posture. He also MTSG's and will join the battle and use the bonuses from Terrain and Weather in the combat but lack any bonuses from entrenchments.
Thus if the terrain is open country, referred to as "clear", and thus gives no penalties or bonuses in combat; it may be more beneficial to put a highly offensive skilled General on Offensive Posture to be available to MTSG and use his offensive skill decisively in battle.