text: Appendix B
  affected unit or the fires are allowed by a permissive FSCM, such as a CFL. These restrictions apply to
  conventional and special munitions and their effects. For more information on boundaries, see ADP 3-90.
  Figure B-13. Boundaries example
  PHASE LINE AND TRIGGER LINE
  B-52. A phase line is an easily identified feature in the operational area utilized for control and coordination
  of military operations (JP 3-09). Phase lines (see figure B-14) are usually easily identifiable from a ground
  or air vantage point and may include features such as ridgelines, tree lines, hilltops, roads, and rivers. Phase
  lines can be used to identify limits of advance and to coordinate fires.
  B-53. A trigger line is a phase line located on identifiable terrain used to initiate and mass fires into an
  engagement area at a predetermined range (FM 3-90). The commander designates a phase line as the trigger
  line for FS attack assets. The trigger line's location is based on such variables as the time of flight for artillery
  shells, positioning of the guns, and the existence of quick-fire links. A trigger line's location varies from
  situation to situation. Its position reflects the distance that the enemy force is likely to traverse in the time it
  takes from when fires are requested to when artillery rounds impact (see figure B-14). This gives time for the
  FS delivery systems to respond to the initial call for fire. Fires unit commanders must determine the
  appropriate planning response time for their unit based on their weapon system and training proficiency to
  recommend the placement of a trigger line.