In Space Marine Chapters that follow the Codex Astartes, Scouts are Space Marines in training. They have not yet gained the Black Carapace, the gene-seed organ that allows a Marine to interface with his power armour. They are usually organized into squads in the Tenth Company, and are the only Marines that regularly wield sniper rifles or use camouflage (although, in some Chapters and the Deathwatch, full Marines do use both). Fittingly, the Raven Guard use a lot of Scouts, since their combat doctrine centers around infiltration and sabotage. Once he serves about ten years as a Scout, the Space Marine receives his Black Carapace and power armour, and is sent off to the Devastator Squads.
They are led by a Sergeant, who is usually a member of a Tactical Squad that chooses to impart his wisdom to a younger generation, but may also be a Veteran Scout who refused the opportunity to become a full-fledged Space Marine. In the case of the Blood Ravens, they are led by Steve Blum voicing an angsty, ineffectual dork, who he hopes one day to teach all the Blood Ravens to be angsty, ineffectual dorks. His most important lesson is to go out of the way to point out problems and question every solution as if it was the most inexperienced idea you've ever heard.
Back in the times of the Great Crusade however, the Space Marine Legions didn't have Scouts at all. Instead they had the even better Legion Reconnaissance Squads. Instead of being made out of simple Neophytes that were to experience their tour of actual combat, Legion Reconnaissance Squads were made out of experts at reconnaissance, intel gathering, guerilla warfare, assassinations and sabotage. If they were fighting along their brethren on the battlefield, then they'd become flankers and wreck havoc and confusion among the enemy's ranks. The Primaris would revive this concept with Eliminators.
A history of stat lines
So how mighty is the Scout, as Junior Marine, on a stat line basis? Well, he has always gotten slightly less good armour, due to not power armouring. In codex Imperialis and Codex Ultramarines, Scouts had M4 WS4 BS3 S4 T3 W1 I4 A1 Ld7, which made then as good at punching as Space Marines (assuming 4 all besides W and A was the space marine), but not as good at taking punches or shooting shit. In 5th edition, Scouts traded their Weapon Skill for their Toughness, and probably got a buff to leadership. Making them guardsmen when it comes to hitting things but space marines in the wounding. By 6th edition, all scouts/claws had space marine muscles, but had the finesse of a guardsman. When the Space Wolf and Blood Angel codices continued the trend, everyone expected this to hold through the edition. Then the Space Marine Codex dropped. At no point increase on anything, scouts and anything scout related got WS/BS 4. The Dark Angel codex dropped as well, with the same stat line boost as the Space Marine codex, and free access to Grim Resolve (i.e. Chapter Tactics for Dark Angels). As of current with all 7th edition codex released, half of the Adeptus Astartes books have junior Marines with WS/BS 4. This may be fair for Space Wolves, because blood and sky claws get Rage. It's still full of derp, as while the matching S and T make sense (the black carapace they lack would let them keep abilities like Move Through Cover in Power Armor if Gee Dubs bothered implementing it), by definition they have way less experience than non-newb Marines and should be worse at getting shit done.
All of this is of course forgetting that in 4th Edition Scouts had WS/BS of 4 to go along with S/T 4, so if anything 5th Edition was a nerf and 7th restored them to their rightful place of being useful. To go along with the fact that a WS/BS of 4 actually makes sense for a group that doesn't get to stop being Scouts until a slot opens up in a Company.
I Can't Believe It's Not A Scout™
- Space Wolves Wolf Scout: Scouts in the Space Wolves are bitter, experienced veterans taken from the Grey Hunters (the Space Wolves equivalent of the Tactical Marines); most of them are introverts that find it hard to fit in with the rest of the (incredibly gregarious) Space Wolves and prefer to operate more-or-less alone. New recruits are put in Blood Claw squads instead.
- Black Templars Neophytes: The Black Templars are too cool to use Scouts. Instead, when an Initiate (the Black Templar equivalent of a Tactical Marine) decides he has learned enough, he takes on a Marine-in-training, referred to as Neophytes, which are then mixed into Crusader squads. This is nominally to pass the experience of an Initiate to a Neophyte, but in reality, the Neophytes just make amazing meatshields. And weapons caddies.
|Forces of the Blood Angels|
|Command:|| Apothecary - Brother-Captain - Brother-Sergeant - Chaplain |
Chapter Master - Librarian - Sanguinary Priest - Techmarine
|Troops:|| Assault Squad - Devastator Squad - Sanguinary Guard - Scout Squad |
Tactical Squad - Terminator Squad - Veteran Squad
|Death Company:||Death Company - Death Company Chaplain - Death Company Terminator|
|Walkers:|| Contemptor-Furioso Dreadnought - Death Company Dreadnought |
Dreadnought - Furioso Dreadnought - Librarian Dreadnought
|Transports:||Drop Pod - Razorback - Rhino|
|Vehicles:|| Baal Predator - Land Raider (Land Raider Crusader - Land Raider Phobos |
Land Raider Redeemer - Land Raider Angel Infernus)
Predator Tank - Vindicator - Hunter - Stalker - Whirlwind
|Flyers:||Stormhawk - Stormraven - Stormtalon - Storm Eagle - Thunderhawk|
|Allies:||Space Marines - Primaris Marines|
|Forces of the Dark Angels|
|Command:|| Apothecary - Company Master - Interrogator-Chaplain |
Chaplain - Librarian - Techmarine - Deathwing Knight
Dark Angels Grand Master
|Troops:|| Assault Squad - Deathwing - Company Veterans |
Devastator Squad - Scout Squad - Tactical Squad
Ravenwing Black Knight - Watcher in the Dark
|Walkers:|| Dreadnought (Deathwing Dreadnought - Contemptor Dreadnought) |
Contemptor-Mortis Dreadnought - Mortis Dreadnought
|Transports:|| Land Raider (Land Raider Phobos - Land Raider Crusader |
Land Raider Redeemer - Land Raider Ares
Land Raider Solemnus Aggressor) - Rhino - Drop Pod
|Vehicles:|| Bike Squad - Razorback - Predator - Vindicator - Hunter |
Stalker - Whirlwind - Land Speeder Vengeance
|Flyers:|| Dark Talon - Nephilim Jetfighter - Stormraven - Storm Eagle |
|Allies:||Space Marines - Primaris Marines|