text: Chapter 6
  6-201. Commanders determine the location, orientation, and depth of the security area based on the terrain
  available and the purpose of the defense. Units identify enemy avenues of approach and named areas of
  interest. The depth of the security area determines the time available to react to approaching enemy forces.
  Occupying a deep security area allows the security force to destroy enemy reconnaissance assets without
  compromising critical observation posts or positions. It prevents enemy forces from easily penetrating the
  security area and prevents gaps when observation posts or units displace or are lost. Security forces cover
  less depth along a broad front because they have fewer resources to position in depth and potentially fewer
  opportunities to exploit terrain. Shallow security areas may require units to task-organize and resource their
  security forces to perform guard operations that provide more reaction time for the main body.
  6-202. The task organization of the security force depends on its role in the overall concept for defense.
  Covering forces and guard forces require more reinforcement than screening forces. Fires, engineers,
  aviation, and other attachments increase the ability of guard or covering forces to slow and disorganize the
  enemy, degrade the enemy’s security forces, and gain additional time for the defending commander.
  6-203. Commanders determine when to occupy security areas based on their assessment of all possible
  enemy courses of action and the preparation required in the main battle area. They allow enough time for
  their security force to move and establish its positions. The mission variables influence how these security
  forces deploy and occupy positions. Commanders may push forward to establish a security area, or they may
  establish one along a current forward line of troops while preparing the main battle area further to the rear,
  depending upon the favorability of the terrain and enemy dispositions.
  6-204. Information collection capabilities enable the performance of security tasks. Commanders focus
  their collection on named areas of interest to detect and track enemy forces. Data and reporting from UASs,
  intelligence operations, ground sensors, and national assets can expand the areas under surveillance, provide
  early warning, and cue security forces. Rotary-wing aircraft detect and report enemy forces at extended
  ranges with sensors. This permits security force units on the ground to concentrate their forces on likely
  enemy avenues of approach, named areas of interest, targeted areas of interest, and restrictive terrain that
  degrades sensor performance. Commanders employ their information collection assets to detect enemy force
  movements, which provides time to reposition security forces and employ assets to counter enemy actions.
  6-205. The security force’s rear boundary is normally the battle handover line. Handover of the battle from
  forces in the security area to forces in the main battle area requires close coordination to avoid confusion and
  fratricide. The security force must retain freedom to maneuver to conduct a rearward passage of lines. Main
  battle area forces establish contact points, passage lanes, and routes through the main battle area. Control of
  indirect fires passes to main battle area units as the security force moves through the passage lanes. Typically,
  battalion-sized units of a security force hand the battle over to the brigades through which they pass.
  Regardless of size or echelon, they must be able to alert the main battle area commander that the enemy is
  approaching on particular avenues of approach and keep the enemy’s leading units under observation. After
  passage, security area forces move where directed and begin preparing for subsequent operations.
  MAIN BATTLE AREA OPERATIONS
  6-206. The main battle area is where the commander intends to deploy the bulk of the unit’s combat power
  and defeat an attacking enemy force. The commander positions forces in the main battle area to block enemy
  penetrations, choosing terrain that puts enemy forces at the greatest possible disadvantage. Commanders
  assign sectors to subordinate forces in the main battle area as a means of controlling subordinate maneuver
  unit operations. Defensive sectors should align with major enemy avenues of approach. The force responsible
  for the most dangerous sector is typically the main effort. Commanders employ their reserves in the main
  battle area to halt an enemy attack, to destroy penetrating enemy formations, or to counterattack to regain the
  initiative.
  6-207. Restrictive terrain, choke points, and natural obstacles such as rivers usually favor an area defense
  oriented on key terrain and avenues of approach. Open, less restrictive terrain usually favors a mobile defense,
  which orients on the enemy. Most defenses at the division and corps levels offer opportunities for a
  combination of mobile and area defense. Corps and division commanders maintain situational awareness
  about close operations in the main battle area while maintaining focus on deep operations to prevent enemy
  forces from reinforcing or effectively supporting their forward echelons.