text: Fire Support Coordination Measures
  Figure B-14. Phase line and trigger line example
  B-54. The commander can establish another trigger line for the most accurate long-range weapon system in
  the vicinity of the area where the FS impacts to capitalize on the asymmetric attack. However, dust and debris
  resulting from the artillery fire may prevent the direct fire systems from engaging the enemy. The commander
  establishes other trigger lines for shorter-range systems. The commander may give guidance to extremely
  proficient crews to engage the enemy at longer than normal ranges or give them different engagement
  priorities than the rest of the force, such as giving priority to engaging air defense or engineer-breaching
  systems. This could result in losing the effect that the sudden application of massed fires has on an enemy.
  B-55. When the enemy reaches these closer trigger lines, the commander establishes a decision point to help
  determine should longer-range systems continue to fire in depth or to concentrate fires on a single point.
  Many factors impact the decision, most of which concern the enemy and how they maneuver and the effects
  of the defending force's fires.
  B-56. The purpose of each phase line and any actions required by forces affected by the phase line will be
  specified on the OPORD of the establishing HQ. Any commander given an AO can establish a phase line.
  For more information on phase lines see ADP 3-90.
  POSITION AREA FOR ARTILLERY
  B-57. A position area for artillery is an area assigned to an artillery unit to deliver surface to surface fires
  (FM 3-90) see figure B-15 on page B-20. The supported commander assigns PAA as a terrain management
  technique. A PAA potentially attracts enemy counterfire so other units should stay away from that area to
  avoid enemy artillery attacks. The exact size of a PAA depends on the mission variables of METT-TC (I). A
  Paladin platoon normally requires a PAA encompassing over four square kilometers, and a MLRS platoon
  requires twelve square kilometers. Units equipped with M119A3 or M777A2 requires one square kilometer
  PAA.