text: Tactical Control Measures
  Sector of Fire
  A-83. A sector of fire is the area assigned to
  a unit or weapon system in which it will
  engage the enemy in accordance with
  established engagement priorities. Battalions
  and smaller echelons primarily use this direct
  fire control measure. Each sector of fire can
  extend from a firing position to the maximum
  engagement range of the weapon, or it can be
  an enclosed area at a distance from the firing
  position. Commanders assign each subordinate
  unit or available weapon system a primary
  sector of fire and a secondary sector of fire to
  increase the capability of concentrating fire in
  certain areas. The primary sector of fire is that
  area in which the assigned unit, individual, or Figure A-40. Sectors of fire
  crew-served weapon is initially responsible for
  engaging and destroying enemy targets located in that sector based on established priorities for engagement.
  Fire shifts to the secondary sector on order when there are no targets in the primary sector or when the
  movement of another friendly element needs covering. This secondary sector of fire should correspond to
  another element’s primary sector of fire to obtain mutual support. Subordinate commanders may impose
  additional fire control measures as required. Figure A-40 depicts primary, secondary, and enclosed sectors
  of fire.
  Target Reference Point
  A-84. A target reference point is a predetermined point of
  reference, normally a permanent structure or terrain feature that
  can be used when describing a target location (JP 3-09.3). A
  target reference point (TRP) is an easily recognizable point on
  the ground, either natural or manmade used to initiate, distribute,
  and control fires. Maneuver leaders at battalion and lower
  echelons designate TRPs to define unit or individual sectors of
  fire and observation, usually within an engagement area. A TRP
  can also designate the center of an area where a commander
  plans to rapidly distribute or converge fires. Leaders designate
  TRPs using the standard target symbol and numbers. Once Figure A-41. Target reference
  approved by the echelon fire support officer, TRPs can be point 032
  designated as indirect fire targets by using the standard target
  symbol and target numbering identification (using two letters and four numbers). If a TRP is not also used as
  an indirect fire target, it is designated using numeric marking only. Figure A-41 depicts the symbol for TRP
  032, a direct fire only TRP. The rest of the TRPs in the figures in this publication are both direct and indirect
  fire targets and are thus designated using indirect fire procedures.
  Trigger Line
  A-85. A trigger line is a phase line used to initiate and mass fires into an engagement area or an
  objective at a predetermined range for all or like weapon systems. Event or time-oriented criteria are
  used to initiate planned actions directed toward achieving surprise and inflicting maximum destruction on
  the enemy. Commanders can designate one trigger line for all weapon systems or separate trigger lines for
  each weapon or type of weapon system. Commanders specify the engagement criteria for this situation. The
  criteria may be either time or event driven, such as when a certain number or certain types of vehicles cross
  the trigger line before initiating engagement. Commanders can use a time-based fires delivery method or a
  geography-based fires delivery. Commanders may reserve the authority to initiate engagement by firing the
  commander’s own individual weapon or giving the command to fire.