Since I started purchasing and selling automotive parts 20+ years ago, I've learned a lot about how the supply chain works for performance parts. But working with various vendors, factories, exporters, resellers, importers, distributors, and brokers, there ends up being a lot of other more complicated labels and stories about how to source parts for your car.
So with that said, I've put together some info in hopes of providing some clarity for people wanting to buy parts. It's broken down into major categories that you may or may not be familiar with.
To start, let's clarify some terminology:
Original Equipment Manufacturer (OEM) - this is simply a blanket term for any factory that produces "original equipment". This OEM factory can be a maker of genuine branded parts (i.e. genuine parts you would buy from a Nissan or Toyota dealer), or it can be a factory that makes parts for any type of branded parts that you might buy from an auto parts store.
Aftermarket - To put it simply: aftermarket encompasses anything not installed on the car on the factory assembly line. In practice, aftermarket is just generally used to describe parts not branded as genuine. Usually in layman's term, this can mean one of two things: aftermarket replacement parts, or aftermarket performance parts. The two are not the same... "aftermarket replacement" simply means some part that will fit and function as the same as the original part you are trying to replace. It may or may not work better, that is up to the seller to disclose. "Aftermarket performance" generally refers to parts you use to modify/upgrade your car for increased performance. The part itself usually is manufactured with higher quality parts, but at the expense of NVH.
Genuine OE Parts
These are usually parts you buy from a dealer that also sells the car that carries the brand. For example, "genuine OE Nissan parts" usually mean you purchase them at a parts counter at a Nissan dealer. In the case of parts for cars not sold originally in the US, it's brought in via their internal parts distribution system or via an authorized 3rd party. Such is the case with Nissan, most of the non-US parts are imported via Nissan Motorsport (which is not the same as Nissan Motorsports International aka Nismo in Japan).
Many times, you will come across older cars with "discontinued" parts, or no longer available. Here's what happens during the typical life cycle of a part:
For cars still in production, parts are made by a third party manufacturer that specializes in that type of part. Springs are made by vendor A, shocks are made by vendor B, bushings are made by vendor C, the list goes on...
Usually third party vendors have a contract with the vehicle manufacturer that stipulates them to continue making a part for X amount of years after a car has stopped production.
After the contract expires, it is at the discretion of the vendor to keep producing parts or discontinue it, based on their own cost structure.
Some parts in smaller territories may be produced by authorized OE replacement parts vendors. They might not be the same vendor as the original maker of what you are looking to buy, but the part will be identical in size and quality as if you were buying from Nissan's original vendor.
Once a part number is flagged as discontinued/EOL (End of Life), the part itself is no longer orderable from the parts system you are inquiring from. So if a part is discontinued within Nissan North America, you cannot order it from a Nissan dealer in North America. But if it's still orderable within Nissan Japan's system, then you can buy via gray market from Japan.
Sometimes it's possible to get parts that are listed as discontinued, because it is still available from authorized vendors that can reproduce the part. It is branded as genuine only because it is still available if that brand ordered new parts in that region.
OE Replacement Parts
These are often mixed in with the OE aftermarket parts listed below, but it's closer to the genuine OE parts explained above. The parts are identical to the genuine OE parts, but often at a slightly cheaper price. The only real difference is 1) lack of genuine parts branding/packaging, and 2) price.
A good example of this are spark plugs you can buy at auto parts stores. They range from factory equivalent spark plugs, to fancier ones with higher performance. They are all OE replacement parts.
OE Aftermarket Parts
The terminology "aftermarket" does not necessary denote performance; rather, it's any part that is a replacement part compared to the genuine or OE replacement , but may have different components that are different than what was installed on your car to begin with.
Usually, a supplier that is labeled OE aftermarket is a supplier that is not some type of OEM. It's a bit confusing but usually OE aftermarket is something used by importers or distributors... it ends up being folded into OE replacement or aftermarket performance parts on the retail level.
Aftermarket Performance Parts
Aftermarket performance parts are what the car enthusiast is familiar with: parts that modify one or more characteristic of the vehicle, to increase performance of the part that is replaced. It can be engine parts, chassis enhancements, suspension improvements, or anything that "improves" the car. Usually aftermarket performance parts are a direct replacement of an original part from the car, and have one or more high performance attribute to it that is ideal for extracting more performance out of the car.
Examples of this would be aftermarket suspension arms, which have a pillowball or solid bushing for precision control, and adjustments built in so you can have a wider range of alignment adjustment.
Another would be engine internal parts such as a metal head gasket. Compared to the factory paper gasket on some engines, it provides superior sealing capability between the engine block and cylinder head, allowing for higher combustion chamber pressures via higher boost levels and lets you make more power with the same displacement.
OE replacement, OE aftermarket, and aftermarket performance parts all share the same attribute: manufacturers will continue to make them as long as there is a demand for the parts in the market. The key difference is that OE parts are usually sold from the factory/vendor directly to an importer/distributor, while aftermarket parts follow a different distribution route.
Many performance brands do not produce a lot of their parts in-house due to the expensive cost of tooling, so the parts they sell are limited by the requirements of the factory or vendor that is helping them manufacture. The flow of parts ends up being this:
Factory > Brand owner > Importer/Distributor > Retail shop
I hope this provides you with some information about parts for your car and how it is differentiated. More importantly, when someone says they have "genuine OE" parts, there's a long and complicated story of where it is produced and how it arrives to your doorstep.