Some creatures are utterly fearless and will never give up a battle, no matter how hopeless the situation might be. This could be because of the troops' extreme bravery, because of a magical effect or because the creatures fighting are not truly self-aware.
Unbreakable troops are(see above). In addition, if defeated in close combat (even by -causing creatures that outnumber them), Unbreakable troops continue to fight on regardless of results. They automatically pass all , regardless of modifiers, can never flee and will literally die fighting under any circumstances.
Units classed as Swarms in their special rules in the relevant Army book represent seething masses of small creatures, often summoned to the battlefield by way of magic. They are an unthinking mass, completely devoid of any self-awareness, and are consequently Unbreakable.
On the other hand, when swarms start to lose a fight; it's easy for their enemies to inflict lots of damage upon them (mostly by stomping all over the little critters!) swarms that are defeated in combat immediately take a number of wounds equal to the difference they've lost the fight by. These additional wounds cannot be saved, not even by ward saves, nor can they be regenerated. For example, a Rat Swarm loses a fight by 3 points. The unit immediately suffers a further 3 wounds.
For example, a Rat Swarm loses a fight by 3 points. The unit immediately suffers a further 3 wounds.
Note that swarms who are subject to special combat result rules (like Undead and Daemonic swarms) do not have the Unbreakable rule, but use their army's own special rules instead ('Undead' and 'Daemonic Instability' in the examples above).