text: Appendix A
  Final Protective Line
  A-76. A final protective line is a selected
  line of fire where an enemy assault is to be
  checked by interlocking fire from all
  available weapons and obstacles.
  Whenever possible, units reinforce the final
  protective line (FPL) with protective
  obstacles. All company and below echelons
  identify their final protective lines as part of
  their defensive preparations. Typically,
  initiation of FPFs is the signal for all
  elements to shift fires to their assigned
  portion of the final protective line and spare
  no ammunition in repelling the enemy
  assault. Figure A-36 depicts a FPL.
  Figure A-36. Final protective line
  Forward Edge of the Battle Area
  A-77. Forward edge of the battle area is
  the foremost limits of a series of areas in
  which ground combat units are deployed to
  coordinate fire support, the positioning of
  forces, or the maneuver of units, excluding
  areas in which covering or screening forces
  are operating (JP 3-09.3). A FEBA is not a
  boundary, but a phase line designating the
  forward most point of the MBA and helps
  convey the commander’s intent. It marks
  the foremost limits of the areas in which
  most ground combat units deploy,
  excluding the areas in which security
  forces are operating. MBA forces can
  temporarily move forward of the FEBA to
  expedite the retrograde operations of
  security forces. A commander designates a
  FEBA to coordinate fire support and to
  Figure A-37. Forward edge of the battle area
  help maneuver subordinate forces. The
  FEBA shows the senior commander’s planned limit for the effects of direct fires. Defending units must
  address this area in their scheme of maneuver and exchange information regarding tactical plans at contact
  points. The Army only uses a FEBA during defensive operations. Figure A-37 depicts the current FEBA and
  a proposed FEBA.
  Main Battle Area
  A-78. The main battle area is the area where the commander intends to deploy the bulk of their unit to
  defeat an attacking enemy. The bulk of a unit’s combat power is deployed in MBA. The MBA extends
  from the FEBA to the unit’s rear boundary. Security forces are located outside of the MBA.
  Sector
  A-79. A sector is an operational area assigned to a unit in the defense that has rear and lateral boundaries
  with interlocking fires (FM 3-0). By not emplacing a boundary to a unit’s front it allows them to synchronize
  fires better against the enemy they are facing. A sector is different from an area of operations in that instead