text: Appendix A
  Figure A-33. Brigade assigned area with battalion battle positions
  Disengagement Line
  A-73. 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 (ADP 3-90). Commanders use
  these lines in the delay and the defense when they want their defending units to avoid becoming decisively
  engaged. They establish criteria, either time or condition based, for disengagement, such as number of enemy
  vehicles by type, friendly losses, or enemy movement to flanking locations. They may designate multiple
  disengagement lines, one for each system in the defense, or if they want the unit to conduct multiple
  displacements. Figure A-34 depicts PL JOAN as a disengagement line.
  Engagement Area
  A-74. An engagement area is an area where the commander masses effects to contain and destroy an
  enemy force. This includes organic direct fire systems and supporting systems, such as close air support.
  The size and shape of the engagement area is determined by the relatively unobstructed intervisibility from
  the weapon systems in their firing positions and the maximum effective range of those system. Commanders
  designate EAs to cover each enemy avenue of approach into unit positions. The commander routinely
  subdivides their EA into smaller EAs for subordinates using one or more target reference points or by
  prominent terrain features. While subordinates are assigned sectors of fire, responsibility for an avenue of
  approach or key terrain is never split. Commanders at battalion and below typically use this control measure.
  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. Figure A-34 depicts a
  sketch of several engagement areas used within the context of a reinforced battalion task force defense.