General Sanchez is a high-ranking UNIT officer. He was command of UNIT when the Daleks invaded the Earth in 2009. He appointed Martha Jones, the Medical Director of Project Indigo which a teleportation system developed from Sontaran technology. When the Daleks invaded UNIT's New York Base, Sanchez and another soldier fended off the Daleks as Martha escaped by means of Project Indigo. Sanchez was last seen firing a gun at a Dalek and he was presumably exterminated.
He probably ranked above Colonel Mace as it was a promotion to Sanchez's office. He is likely the overall head of UNIT whereas Mace is the head of the UK division.