The A05.3 was the one of the combat armor suits used by the Sith Order

Sith Order Organization Reconstituted Sith Empire

during and after the Great Galactic War

Great Galactic War Event Great Galactic War (3681BBY - 3653BBY)

.
The Sith Order was less regulated in its choice of armor and uniforms than the Imperial Army

Imperial Army (Sith Empire) Organization Reconstituted Sith Empire

, but to some extent its members were also required to wear a certain type of armor while on mission. This set of armor was provided to the warrior

Sith Warrior Organization Galaxy-Wide Organizations

's branch of the Order, but was usually used only by lower-ranking members, such as adepts or acolytes.
The basic set provided light protection to the wearer's body, legs and arms. Its design allowed to use an advanced components such as heavy boots, extra chest armor plate, arm bracers and shoulder pads.