True Damage
This type of damage is only dealt by specified effects, such as overfatigue damage. True damage cannot be prevented or reduced by abilities unless they specifically affect true damage. True damage ignores any bonus HP and reduces the unit's HP instead.
All other damage is physical damage; see DamageDamage and Preventing DamageDamage is physical unless it's true damage. All damage dealt to a unit during an action is a single instance. Prevented damage is subtracted 1 point at a time; it was still dealt, but not taken.Rulebook p. 63.
If an ability avoids true damage, can I use it to avoid normal damage instead?
No, damage and true damage are different types of damage, rather than one just being a stronger form of the other. If something specifically avoids true damage, that doesn't mean it can avoid normal damage.