I find it disappointing that there's very few official armor permutations for the Space Marines given the extensive autism behind the lore of each chapter. I mean era specific stuff. Since the Primaris Marines are different than the ones that served during the horus heresy.
Its largely just a recoloring of the same handful of units outside of a few OCDONUTSTEEL badasses like psykers that are relevant to the lore with the alternate helmet they have. You'd think that the chapters more focused on fighting specific factions like Chaos would have iterated upon their chapter's armor to adapt to the threats they specialize in fighting.
Meanwhile Halo 3 had at least 7 for its Spartans and even more for Halo Reach that all had a simple flavor text paragraph quickly explaining where it was developed and what problem or role each armor piece was made to help with. At least there's always making custom miniatures.
Well, it gets odd, and now I'm going to autism at you for a bit. Sorry if this is old news.
There are the various Marks/Makes of the armor, showing generational changes and changes for specific problem/roles. 1 through 8, or Thunder, Crusade, Iron, Maxiumus, Heresy, Corvus, Aquila, and Errant, each showing, generally, a generational evolution of the armor, at least until 7 and maybe 8 when the static nature of the setting hits hard.
Thunder armor was more for the Thunder Warriors, the proto-Space Marines that the Emperor would prefer you not ask about. (Once they were no longer useful to him, and he had created less powerful but controllable replacements, he betrayed and killed them all.) I don't think any Space Marines used it.
Crusade and Iron armor were mostly Horus Hersey armors, and are old relics by 40k's or even HH's timeframe, but are often used by certain chapters or Chaos Space Marines. They're big chunky boys, very distinct and cool looking. Crusade Mark 2 was designed for general use, being basically a heavily armored space suit, IIRC Iron Mark 3 was more for ship to ship boarding actions and was thicker but significantly harder to move in.
Maximus, Mark 4, armor kinda looked like modern 40k armor, and is the "old" style of "modern" armor. There were a lot of variants of it, or at least the helms (as that's easier to put on a spare sprue), mostly for muh special snowflake chapters.
Heresey, Mark 5, armor was the one with dots on the legs/shoulderpads, and was meant to be kitbashed in universe; scrap armor put together -- the dots on the legs/arms/whatever are literally spaceship sized rivets that they're using to hold together the armor after repairs. As the war went on they were literally taking bits of armor off dead dudes and using Ork engineering to keep it going. In theory it's supposed to be cracked, torn apart, with pieces of the equivalent of futuretech sheet metal bolted down under those dots, but, that would have been a bit much to model at the time, so...
Corvus (Mark 6) is the infamous "Beaky" armor and used to be more commonly seen in 40k, especially early 40k, but with NuHH's launch they decided to make it the semi-official HH Armor to contrast with Primaris. It was meant to replace Mark 4 armor, at least temporarily, but due to fuckery of the Alpha Legion and Horus, the only ones that really used it were the Raven Guard, and no one remembers them.
Aquila, Mark 7, armor was developed during the Heresey and shipped out to the loyalists literally as Mars fell to the invaders. It's the one with a huge chest eagle. It's technically a variant of Beaky armor, and is "40k loyalist space marine" armor.
Errant armor, Mark 8, is mostly a Deathwatch thing, since it was just getting rolled out in the 40k era when the timeline froze and Deathwatch get all the fun toys, it's the one with an armored collar. Oh, the Minotaurs, too, cause in lore they're being given embezzled shit as they're role as the High Lords of Terra's little enforcer pricks. More often than not you'll see a Mark 8 chest piece used for a HQ unit's armor to make them stand out, which is useful cause they require special helmets, normal helmets won't fit.
So, basically, Crusade armor is the generic stuff, Iron armor was created to be tougher but was less mobile, Maximus Armor was replacing all previous armors (with plans on replacing THAT with Terminator armor for every single marine eventually), Hersey Armor was them desperately fixing Maximus Armor with literally anything they could find, Corvus Armor was to be a holdover until they could do a full refit, Aquila armor was them desperately making a better armor to fight off the Horus Hersey, and Errant is the newer stuff (cause there's a flaw in Aquila armor that will cause bullets to bounce up into your neck) that didn't get created much cause everything's been static for 10,000 years now in setting.
They skipped Mark 9 -- no doubt for an "in case of needing something cool to unveil, break glass" -- reason, and Mark 10 is the generic Primaris armors, with that itself getting sub-variants.
Back in the day you'd see a lot of mix and mash, and it really added to the setting's feel. So you'd have a Aquilla chestpiece with some MK5 legs, maybe MK4 arms. But that's a huge pain in the ass to model, and it gets in the way of doing portfolio pieces like Primaris Lieutenants in a Dynamic Pose for the 3d Modelers that go to work for GW, get some resume experience, and leave. So they kinda stopped that.
In setting it'd make sense that some Forge Worlds would have regional variations of armor, but canonically, the Emperor made damned sure they all shared the same STL files, so they all were making it pretty much to spec. This was before the HH, which also caused the Forge Worlds to go all civil war, "burn down the library cause where we're going we won't need science" on everyone, so they were more willing to actually innovate and work on shit, more willing to share the armor designs.
It helped that I'm pretty sure Big E would have killed the shit out of any AdMech leader that tried to tell him "no, this is a sacred armor design, we can't share it with the other forge worlds or marine chapters for mass production, that would piss off the data spirits." He was a militant Athiest and didn't believe in the Omnisiah bullshit, but he didn't have time to stamp that stuff out fully before things the fan, and, whoops. 10,000 years later and the science guys are literally a cargo cult.
You did see something similar with the bolters -- Bolters were one of ~3 variants depending on age, with very minor changes (one variant has a sub-variant with a drum mag, for example), with a 4th variant (Forge World's own, created I'm sure cause of some stupid IP rights fuckery reason), the Tigrus bolter, being a regional version that caught on, apparently.
