During the Aeon of Strife, protoss warriors created personal shields by focusing psionic energy. The protoss later reproduced the effect using psi-field generators. These generators could be installed in machines giving protoss vehicles shields as well. Psionically powerful protoss such as Khastiana can still enhance the power of their shields. Shields must be dropped to make use of some physical contact-based psionic abilities.
Shields points act as another form of hit points and are usually lost first. Unlike regular HP, plasma shields always suffer full damage regardless of the type of attack used against them or the size of the shielded object. For example, a large-sized dragoon's shields will take full damage from concussive damage attacks such as from a vulture.
Shield points have a separate "armor" rating from regular hit points. The base "armor" is 0 and may be upgraded. This applies to both units and buildings. If an attack both depletes shields and damages HP, both shield armor and regular armor are subtracted from the attack's damage. Shield points regenerate slowly but a shield battery may be used to restore them faster.