When you start a battle the attacker can take in more than 8 units to fight. At the end of the battle they must move excess units to an adjacent friendly hex (as long as that hex does't exceed 8 units). What happens if there are no friendly adjacent areas, only empty ones? Are excess units destroyed or do the units move to an empty hex?
You are allowed to retreat into an empty hex if there are no adjacent friendly hexes. You are only required to retreat into a friendly hex if one is available.
Do note that you can only retreat to ONE hex after a battle, friendly or otherwise. That means if the number of retreating units exceeds the capacity of the chosen hex, the remainder are destroyed. They don't "spill over" into a different adjacent hex. Of course, in my experience it's highly unlikely that you'll have more than 16 surviving units after a battle (forcing more than 8 units to retreat out of the battle hex) so if you are retreating into an empty hex, you probably don't have to worry about hitting its capacity.