Army Command was a staff of senior generals of the Imperial Army, headquartered on Coruscant. From there, Army Command issued orders to various Army units across the galaxy and kept track of all Army troops movements. It often worked in conjunction with Stormtrooper Command, the leadership of the Stormtrooper Corps. Several Grand Generals were appointed by Command, one of them holding the Chief of Staff position.
It served the Galactic Empire, and later the Imperial Remnant on Bastion. It went on to be used by both the Fel Empire and later Darth Krayt's Galactic Empire. However, under Darth Krayt, it's powers were limited by the Sith, until the Command was eventually dissolved altogether. That infuriated the generals, causing many of them to defect to the Empire-in-exile in 131 ABY.