Marines assigned to the 15th Marine Expeditionary Unit (MEU) Force Reconnaissance Detachment stack up and prepare to enter a room during close quarters combat training aboard Camp Pendleton.
An MEU's Force Reconnaissance Detachment stands ready to carry out 'green side' operations i.e. in-depth reconnaissance on land, as well as 'black side' operations i.e. direct action raids against maritime targets and targets on land.
The foremost Marine, who in this shot is providing rear security, is a Recon Marine with the role of Slack Man.
When operating in a 6-man reconnaissance team on green side operations, the Slack Man is typically the second man in the order of march, one behind the Point Man.
The Slack Man covers the Point Man and watches his flanks.
The Slack Man is usually the junior member of the team.