Ethereal armor usually cast on someone via banish or the Spirit Walkers Ethereal form.
When they're in that form, they take no physical damage. Hence the normal, hero, pierce, and siege factors are all 0.0.
But ethereal armor takes 66% extra damage from magic type (like that of priests) and spells (like death coil or storm bolt or every other offensive spell)
and 66% extra healing from healing spells, hence the factor of 1.66. (Ex: Level 3 holy light heals Ethereal units for 996 instead of 600).
That's about it methinks.
Edit:
Also forgot to mention chaos does 66% extra damage as well hence the factor of 1.66.