Jennison, keep in mind that
cohesion represents a unit’s general readiness for combat. It is critical to a unit’s overall efficiency as it impacts almost every aspect of the game, such as morale, speed, firepower and the ability to “March to the Sound of the Guns”. A unit with low Cohesion could suffer from poor morale, desertions and stragglers, exhaustion, disorganization, inadequate training or any other debilitating factor. It will be slow in moving and prone to break during combat.
Movement and combat reduces a unit’s cohesion while resting gradually improves its cohesion until it ultimately reaches its maximum level. This depends on training and experience. NM affects a unit’s cohesion and its cohesion recovery rate when resting.
On the other hand, as
Longshanks has said wisely

, a
medical service unit is able to provide health care even on the battlefield. It gives +15% to the Cohesion Recovery rate of all others elements of the Stack this unit is in.
Regarding
supply wagons, they are special units which act as moving supply stockpiles and thus provide you with limited direct control over supply. They fill up during the supply distribution phase. Supply wagons allow you to keep selected forces supplied, even if they operate far from their supply bases. However, this will only last until they become empty and need to be resupplied themselves from a regular supply source.
Wagons tend to slow down the force they accompany and cannot be used by naval units.
Supply wagons also have the following additional benefits:
* They provide a +10% fire bonus during battles (provided they have some ammo left).
* They protect units from bad weather effects by trading hits for supply.
A supply wagon or transport ship is able to supply any land unit with both General Supply and Ammunition if it is in the same or an adjacent region.
I hope this can help you to understand the mechanic of the game better.
Enjoy Playing!
