1-21. Sustainment maintenance is performed by Department of Defense (DOD) civilians and contractors who return equipment to a national standard, after which the equipment is placed back into the overall supply system. The United States Army Materiel Command (USAMC) through the Army Sustainment Command (ASC) and its subordinate Army field support brigades (AFSBs) and Army field support battalions execute contingency sustainment maintenance missions. When a unit sends equipment to a sustainment maintenance organization the owning unit, in most cases removes the equipment from the property book. Only in rare instances such as unit reset, will the equipment be returned to the unit. Sustainment maintenance is composed of two subcategories; below depot-level sustainment maintenance and depot-level sustainment maintenance. Units utilize sustainment maintenance when crew, operator, operator-maintainer or Ordnance Corps trained maintainers lack the requisite skills, special tools, proper repair parts, or references to complete repairs using field maintenance. Based on the extent of damage to the specific item, leaders must decide the best course of action based on operational and mission variables. There is no absolute checklist. Field-level maintenance is the preferred method of repair. The intent of sustainment-level maintenance is to perform commodity oriented repairs to return items to a national standard, providing a consistent and measureable level of reliability. Sustainment-level maintenance supports both operational forces and the Army supply system. See ATP 4-33 for a description of the sustainment maintenance subcategories.