From ErfWiki

Proposed Canon

A stack is any group of same-aligned units in the same hex.

Groups of units get a stack bonus that maxes at eight. This makes eight-man formations fairly common. Stacks without a leader are forced to auto-attack when in contact with units from non-allied capitals. With a Warlord the fight can be directed, or the group can choose not to initiate a fight.^Erf-b1-p040a