ADP 3-90, page 65

The Defense 31 July 2019 ADP 3-90 4-5  A supplementary position is a defensive position located within a unit’s assigned area of operations that provides the best sectors of fire and defensive terrain along an avenue of approach that is not the primary avenue where the enemy is expected to attack. An avenue of approach into a unit’s AO from one of its flanks normally requires establishing supplementary positions to allow a unit or weapon system to engage enemy forces traveling along that avenue.  A subsequent position is a position that a unit expects to move to during the course of battle. A defending unit may have a series of subsequent positions. Subsequent positions can also have primary, alternate, and supplementary positions associated with them.  A strong point is a heavily fortified battle position tied to a natural or reinforcing obstacle to create an anchor for the defense or to deny the enemy decisive or key terrain. Commanders prepare a strong point for all-around defense. Commanders establish a strong point when anticipating enemy actions that will temporarily isolate a defending force retaining terrain critical to the overall defense. 4-26. When assigning battle positions, the commander always designates the primary battle position. Subordinate commanders designate and prepare alternate, supplementary, and subsequent positions as time and other resources permit, and if the terrain or situation requires them. Before assigning a strong point, commanders ensure that the strong point force has sufficient time and resources to construct the position. Constructing battle positions requires significant engineer support and Class I (primarily water for CBRN decontamination and consumption), Class IV (construction materials), and Class V (ammunition) supplies. A minimally effective strong point typically requires one day of effort from an engineer unit of the same size as the unit defending the strong point. Normally, companies and battalions occupy strong points, although brigades may construct them. Commanders do not normally establish strong points for units smaller than company size. This is because a platoon or squad cannot secure a perimeter large enough to contain all required assets and supplies. DIRECT FIRE CONTROL MEASURES 4-27. Commanders engage an enemy force with all available fires when it enters a defending unit’s engagement area. (See FM 3-90-1 for discussions on several direct fire control measures such as target reference points and trigger lines.) DISENGAGEMENT LINE 4-28. A disengagement line is a phase line located on identifiable terrain that, when crossed by the enemy, signals to defending elements that it is time to displace to their next position. Commanders use disengagement lines in delays and defenses when they do not want a defending unit to become decisively engaged. Commanders establish criteria for a disengagement, such as number of enemy vehicles by type, friendly losses, or enemy movement to flanking locations. Commanders may designate multiple disengagement lines, one for type of each weapon system or subordinate unit. ENGAGEMENT AREA 4-29. An engagement area is an area where the commander intends to contain and destroy an enemy force with the massed effects of all available weapons and supporting systems. In the defense commanders shape the enemy approach and steer enemy formations into engagement areas. They then concentrate overwhelming combat power in a focused area to defeat an enemy attack. The seven steps of engagement area development are identify all likely enemy avenues of approach, determine likely enemy schemes of maneuver, determine where to kill the enemy force, plan and integrate obstacles, emplace weapon systems, plan and integrate indirect fires, and rehearse. FIRE SUPPORT COORDINATION MEASURES 4-30. Commanders try to engage an enemy force at extended ranges as the enemy’s attack advances. To control indirect fires, commanders use permissive and restrictive FSCMs. Permissive FSCMs include the coordinated fire line, the FSCLs, and free-fire areas. Restrictive FSCMs include the airspace coordination area, no fire area, restrictive fire area, and restrictive fire lines. (See FM 3-90-1 and FM 3-09 for discussions