"True Damage" is a special damage type which is not Physical nor Magic.

It can be boosted by generic damage buffs.

It can be boosted by a percentage of both Physical Attack and Magic Power.
Do note that in some cases only Physical (or only Magic, or some completely different parameter) applies, check specific Hero ability description (or Stand attack formula) to avoid mishaps.

It can be boosted by unspecified percent of both Physical Crit Rate and Magic Crit Rate (and their corresponding Crit Damage).

Both Heroes (Vampire) and Stands (Monk) can deal True Damage.

True Damage doesn't need dedicated Penetration stat since it doesn't seem to be affected by Armor.

It behaves as any usual damage type otherwise. Just double-dips into some stats and makes Penetration a dump stat.