text: Chapter 13
  13-51. The four types of security operations provide increasing levels of security for the main body.
  However, more combat power in the security force means less for the main body. Screen operations provide
  early warning to the main body. Guard operations prevent enemy observation and direct fire on the main
  body. Cover operations protect the main body from enemy observation and effective direct fire. Area security
  protects friendly installations, routes, units, and facilities within a prescribed area. All types of security
  operations provide protection and early warning to the protected forces, which provides reaction time and
  maneuver space to the commander preserving freedom of action.
  13-52. The screen, guard, and cover operations share many common control measures, starting with
  boundaries defining the security area. The force’s main body establishes the security area. For a security
  force operating to the front of the main body, the lateral boundaries of the security area are normally an
  extension of the lateral boundaries of the main body. The security force’s rear boundary is normally the battle
  handover line.
  13-53. Commanders depict the performance of security operations on overlays using a lightning bolt on
  either side of the symbol representing the unit performing the security operations. They are labeled with the
  letter S, G, or C to denote screen, guard, or cover. The end of the lightning bolt has arrowheads that touch
  the designated control measures, which define the left and right limits of the security operation. Figure 13-4
  depicts guard and screen control measures for the security forces performing those operations.
  Figure 13-4. Example security operations control measures
  13-54. The main body or the security force may designate additional PLs to control the operation. These
  PLs may serve as subsequent screen or delay lines. Each security force element reports to the commander
  when crossing PLs or occupying screen or delay lines. Displacement to these subsequent PLs is event driven
  (enemy or friendly) or time driven. The approach of an enemy force, relief of a friendly unit, or movement