text: Chapter 9
  9-6. Maneuver battalion and BCT security forces normally conduct forward screen or guard operations.
  Division and corps commanders may employ a forward covering force. Division commanders may have their
  forward security forces conduct a guard if their parent corps employs a forward covering force.
  9-7. Commanders designate flank security forces to cover gaps on the unit’s flanks. This occurs when gaps
  between units develop during the conduct of defensive operations. A flank screen or guard is critical if an
  enemy avenue of approach into the defended area from the flanks could be uncovered during the defense.
  MAIN BODY
  9-8. Units build their main effort around identified key terrain. A unit’s main effort in an area defense
  focuses on retaining that key terrain by using fires from mutually supporting, prepared positions
  supplemented by one or more counterattacks and repositioning forces. A unit’s main effort normally involves
  close combat since an area defense emphasizes terrain retention.
  9-9. Units normally position their echelon’s main body—the bulk of combat power—in their MBA where
  they want to conduct their main effort. They organize their main body to fix and finish attacking enemy
  forces. Most of the units in a main body deploy into prepared defensive positions within their MBA.
  However, some mobile elements may hide and are ready to deploy where and when needed.
  RESERVE
  9-10. Units’ defensive plans should be able to succeed without use of their reserves. However, their reserves’
  most likely task is to conduct counterattacks based on previously prepared plans. Lower echelon commanders
  use their reserves primarily to conduct local counterattacks to restore the integrity of their defense or to
  exploit opportunities. Senior commanders use their reserves to seize the initiative from enemy forces when
  opportunities to do so present themselves. For example, a corps commander may choose to employ their
  reserve against enemy fire support assets and follow-on forces to limit the enemy commander’s ability to
  mass combat power on the corps main effort.
  9-11. Commanders can assign their reserve a wide variety of tasks to accomplish, and their reserves need to
  be flexible enough to perform other tasks. Commanders may find it necessary to commit their reserves to
  restore the integrity of their defenses by blocking enemy penetrations or reinforcing fires into engagement
  areas. Secondary tasks for a reserve may also include—
  * Reinforcing the defense of committed forces.
  * Blocking or containing enemy forces that penetrate friendly defensive positions.
  * Relieving depleted units and providing for continuous operations.
  * Reacting to threats directed against friendly sustainment efforts.
  * Extending the flanks of defending units to prevent their envelopment.
  * Covering retrograde movements.
  9-12. Defending units usually have difficulties establishing and resourcing their reserves because they are
  normally facing enemy forces with superior combat power. Nevertheless, commanders at each echelon down
  to the battalion retain reserves as a means of ensuring mission accomplishment and for exploiting
  opportunities through offensive action.
  9-13. Units may not be able to resource separate reserves in some situations. Therefore, they may constitute
  all or a portion of their reserves from their security forces (after those forces conduct a rearward passage of
  lines through MBA units). If units employ a security force as the reserve for an area defense, they withdraw
  the force so it has sufficient time to occupy the reserve positions, perform the necessary degree of
  reconstitution, and prepare plans for reserve roles. However, this is not the preferred option. Before battle
  handover, the senior commander must state the acceptable risk to security forces or the disengagement criteria
  in quantifiable terms, such as friendly strength levels, time, or event. In this case, after completing the
  rearward passage, security forces move to assembly areas to prepare for their subsequent operations. These
  areas should be free from enemy interference and clear of MBA units, LOCs, and MSRs.