This is something that's not covered in the fluff. It's not something people really care about because... ooh super-evoluntionary giant psychic space locust-dinosaur predators getting shot by gundam wing while vat grown super soldiers created by gandalf the awesome-turned god have a civil war while devilish creatures made of pure emotion from an alternative dimension invade real-space and 10 ft tall walking fungi shoot green lightning at space-elves from a catapult made from a few pieces of two-by-four and a couple of elastic bands, all the while the planet is being destroyed by the bombardments from 20km long battle ships launching warheads which can open miniature black holes, turn all living matter into flammable, explosive gas, and then ignite it in a planet-wide firestorm, before cracking open a city sized hole in the ground to let the molten core of the planet bury everyone in a tide of magma.
Sadly, that's 40K as the writers see it, and few take the effort to properly define scouts.
That said, my views are that;
Someone who has been accepted in the chapter as a trainee is going to go through their version of basic. They are going to be going to classes (these classes might include tutelary engines which automatically impart that knowledge into the mind, or actual study), fitness and combat programs - from a young age (remember - Red Scorpions from day 2 of their existence), proper nutrition (I don't think anabolic steroids are frowned on that much no is injecting testosterone through the eyeball or whatever else they do to bulk up) so that they become the fittest possible.
Consider that the scout is completely unused to war. If you think that someone who has spent 20+ ish hours a day, every day, 7 or more years has been trained in all aspects of warfighting by some of the best soldiers and officers in the entire chapter (veteran sergeants, and the like on the training staff). They will be fitter, faster, more intelligent, better shots, with better equipment than your SEAL Team 6, and more dedicated to their cause.
Then you add in their implants, and you can see how much better they are - the equivalent Schola Progenium Troopers will have been in the Scholam's since they were younger, and they're the guys who call Space Marines to do jobs they can't handle.
A Space Marines ability to spit acid is kind of cool, but the Betchers Gland rarely comes into use - some chapters lose access to it altogether IIRC, and the only mention I can remember of it being used was in Brothers of the Snake, when Priad was bitten by a normally lethal (to human) snake while unarmoured and training on his homeland - this venom was then stored. As well as it being more sensible to wear a helmet at all times, it is simply a reserve weapon should the helmet not be worn/lost and the two are in literal spitting distance of one another, and that the target not be wearing armour.
An Astartes in power armour can make immense feats of strength - in the old Inquisitor specialist game, Space marines in power armour increased their strength by 20% - and considering that they already had strength in the 100's, IIRC, they were very powerful at things like jumping and other feats of strength, more than just combat. I think even the strongest of others was around 60, and bionics to be around 80 or so. I seem to remember one story (I might have been one of Ben Counter's Grey Knights?) where an Astartes sprinted after a vehicle, reaching 60+ ish mph (in power armour).
Outside of that armour, they're still going to be strong - but not massive differences so.
A Scout/Neophyte may or may not be ready to deal with particular tasks. A full blown Waaagh and invasion of Armaggedon is not going to be a good place to test a few 13 year olds in their 4th year of training with the Astartes. Although they're likely exceptional fighters by this stage, they're not exactly going to be much use there. Instead, while at the chapter monastery they may receive an alert that there's an outbreak of fighting or something incredibly inconsequential to the vast scheme of things (due to the scale of 40K, this could be something like an insurgency on a protected world).
This would be perfect opportunity for the young neophytes to gain combat experience within a reasonably safe environment for them. There would obviously be risks, but will teach them similar to a life fire exercise, and how when I was inside, we'd conduct live fire over the heads of those training to deploy and conduct (blank fire) operations and assaults against positions so that we'd know how to react.
While I'm sure that the Astartes would do the same, there's nothing like actually being under fire to know how to react.
It could also be a way of culling the chaff. One who is too stupid to keep his head down is not going to be much use to the chapter. Of course, that doesn't account for luck, but then the Astartes have rarely been about relying on luck, and min-max their chances of success of their operations on the ground anyway.
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