In what cases is the "Under armor" parameter important?

[Docent]

In what cases does the value of the “under armor” parameter come into play? When a soldier is shot from the floor below through an elevator, this is understandable. In any other cases? In case a grenade explodes underfoot? And if a grenade exploded in a step in front of a soldier? Two steps away? If the enemy shoots not exactly from below, but at an angle? When this angle decreases, at what point does the “under armor” cease to matter?

[Nord]

When a unit stand closer than 2 tiless away from explosion epicenter.

[Docent]

Thanks.