A stack is a group of units that fight together and share bonuses. To be stacked together, all units must be from the same side or its allies. A hex may contain multiple stacks.Erf-b1-p049
Groups of units get a stack bonus that maxes at eight. This makes eight-man formations fairly common. Any special bonuses apply to the entire stack.
Stacks without a leader are forced to auto-attack when in contact with units from non-allied capitals. With a Commander the fight can be directed, or the group can choose not to initiate a fight.Erf-b1-p040a
Hexes seem to have a maximum number of units or stacks that can exist within them. Columns seem to form because there are too many units to fit within a single Hex.
The three-hex Column that retook Warchalking for Gobwin Knob contained twelve stacks, so the limit may be four or five allied stacks per hex.