text: Chapter 5
  control nodes to support advance guard operations and the positioning of support and sustainment assets to
  allow seamless logistical support to maintain the momentum of the advance guard.
  5-67. Additionally, the BCT commander identifies a unit to serve as a reserve for the advance guard. A
  reserve is that portion of a body of troops that is withheld from action at the beginning of an engagement to
  be available for a decisive movement (ADP 3-90). By allocating forces to field a reserve, the commander
  preserves the freedom to exploit opportunity or reinforce friendly elements as required. For a BCT conducting
  a guard, the reserve is a maneuver company from one of the maneuver battalions. If the Cavalry squadron is
  performing a guard, BCTs consider providing a maneuver company from a maneuver battalion to serve as
  the squadron reserve so the squadron does not place a reconnaissance asset in a reserve role.
  FLANK GUARD
  5-68. A flank guard protects an exposed flank of the main body. In performing this operation, the flank guard
  operates beyond the assigned zone or sector of the protected force. The flank guard’s responsibility begins at
  the trail element of the advance guard or at the lead combat element of the main body. The flank guard ends
  at the rear of the protected force or lead element of the rear guard. The higher commander tasking the flank
  guard specifies the boundary. (Refer to FM 3-90-2 for more information.)
  Stationary Flank Guard
  5-69. A flank guard for a stationary force performs zone or area reconnaissance when establishing the initial
  security line positions allowing the flank guard to become familiar with the terrain they will defend. Upon
  reaching their initial positions, the flank guard establishes a defense and goes through the steps of engagement
  area development. Once the flank guard makes contact, they continue to defend or delay when necessary,
  according to the commander’s intent. As with the advance guard, the commander must allocate forces to
  serve as a reserve (see figure 5-3).