Battle state defines structure of battlefield, positions of stacks and spells possible to cast.
Battle is unusual structure, as it is avaliable only during combat. Info about current battle is handled by BattleState class in BattleState.cpp
Creature stack in a battle is represented by class CStack defined in BattleState.h file, which also defines stack state, active spells or current count. It's different from stack representing a group of creatures in an army.