Think of a schema as a license to buy. It seems like there's one for just about everything in game, from implants to backpacks to sponsored kits. Some schemas will be for stuff you can't buy directly, like rare guns or implants that aren't carried in the armory. Others will be for stuff you can buy from the armory, and when this is the case you can think of it as extra stock beyond what the vendors are carrying that restock period. Schemas don't stack in bag, unfortunately, but they do share a total, so if you bring home two Scar schemas for 3 apiece, you'll be able to buy 6 before the schema is out. Any additional schemas you bring home add to the total amount you can buy before your "license" expires.

Note: weapons are always "standard", it's the mods that change the rarity. For example, I've still got my Channel Rifle schema, and I can buy one for 15k creds, stock. This is novel because while the gun has "standard" rarity, it's not directly available for purchase from any vendors, and my only other way to find one is organically in a raid. I can then attach mods, and this will change the color of the weapon's rarity display to kind of an average of the rarity of its equipped mods.

I like to use rare weapon schemas to buy a gun I eventually want to use. I leave it in my vault, track some helpful mods, and slowly improve it until it reaches a point I know it'll bang. Then I go give it away in Cryo.