Sector Army/Legends

A Sector Army was a type of military formation used by both the Grand Army of the Republic and the Imperial Army, although the role and composition of these units evolved markedly between the Clone Wars and the Galactic Civil War.

Sector Armies in the Clone Wars
In the Clone Wars, Sector Armies were the main subordinate component units of the ten Systems Armies that formed the military backbone of the Grand Army of the Republic. There were two Sector Armies in each Systems Army, making a total of twenty Sector Armies; each of them was commanded by a Senior Jedi General, and had a nominal combat strength of 147,456 troopers, divided into four Corps (although it should also be noted that many of the Legions which made up the Corps also included "Corps" within their unit names).

For instance, the 2nd Sector Army, commanded by Jedi Master Ry-Gaul, was part of Systems Army Alpha, and numbered among its component units the 327th Star Corps, led by Commander Bly and Jedi Master Aayla Secura. The 4th Sector Army, known unofficially as the "Outer Rim Sector Army", was the parent of the 21st Nova Corps, Ki-Adi Mundu's Galactic Marines.

Sector Armies under the Empire
Under the New Order, the number of Sector Armies rose dramatically; each of the Moffs, the Governors who Palpatine dispatched to oversee the thousand Sectors which the Empire had inherited from the New Republic, bore the title of Surface Marshal of a Sector Army. A standard Sector Army now included four field Armies capable of deploying anywhere in the sector - 774,576 combat troops in a total muster of 1,180,379 personnel - plus a number of autonomous Corps HQs for garrison duty.

Behind The Scenes
Sector Armies were introduced as elements of the Imperial Army in The Imperial Sourcebook, and retrospectively given their Clone Wars back-story only in Star_Wars_Insider #84. Obi-Wan Kenobi's Third Army can probably be identfied as another GAR Sector Army, but there seems to be no specific confirmation of this fact.