Raven Guard Scout.
New members (Neophytes) of a an Astartes Chapter are placed into the 10th Company where they serve as scouts, doing reconnaissance and infiltration behind enemy lines as well as assassination. As the Scouts do not have the Black Carapace implant that allows them to move freely in Power Armour, they instead wear lighter Carapace Armour under Chameoline Cloaks. Most are armed with Sniper Rifles or Shotguns, though some use Bolters. Scouts learn valuable combat skills during their time in the 10th company, and traditionally ‘graduate’ to service in Tactical or Devastator squads.
Non-Codex Chapters have different standards for Scouts; the Space Wolves only allow veteran Battle-Brothers to serve as Wolf Scouts, for example. The Black Templars do not have Scouts at all, instead throwing their Neophytes into battle with their regular troops.