Parts retailers are drawing more attention to brands more than ever before. There is the strong desire to avoid "cheap Chinese crap" and only use German-made pieces. This is the wrong outlook and customers should be focused on the quality and reputation of the parts brands rather than where they were made. BMW produces very few components themselves, basically limited to large metal components like engine blocks and complex composite pieces like the i3's carbon fiber chassis. BMW buys most of their parts from other sources, referred to in the industry as Tier 1 suppliers. Examples are Bosch, Continental, and Pierburg. And these parts may be made anywhere in the world, from Mexico to Turkey and even China.
Tier 1 suppliers may be allowed to sell the products to the general public through distributors. This depends on who owns the part design and what their contract is with BMW. BBS may produce wheels for BMW but cannot sell them because BMW owns the design. However, Bosch can sell a spark plug because it's their own intellectual property.
I consider there to be 6 groups of BMW parts suppliers:
OE or Genuine BMW
The part stamped with the BMW logo, with a BMW part number, in BMW packaging, and sold through a BMW dealer. This may not mean BMW produced the part in one of its factories.
OES (Original Equipment Supplier)
An OEM part supplied to BMW with the BMW logo, but is then offered for sale outside the BMW dealer network. This is usually surplus inventory. BMW used multiple suppliers to ensure a consistent parts supply but they have extra parts leftover. In the OES case, the BMW logos are ground off and removed and generic packaging is used.
OEM (Original Equipment Manufacturer)
The original manufacturer of the part, such as Continental for serpentine belts and hoses. As an OEM they usually have a contract that lets them sell the same part under their own brand. It's the same as an OE/OES part but with no BMW markings. In addition, for a part to be labeled "OEM" in Germany it must achieve German TUV certification.
OEM-quality, OEM-grade, or OEM*
When is an OEM part not OEM? This would be a part from an OEM but was not the OES to BMW. The OES for a N54 spark plug is NGK but if Bosch manufactures an identical plug is it not the same quality? Is it not "OEM quality"? I hesitate to label the Bosch plug an aftermarket part even though it fits the definition.
Aftermarket
A part that is copied or duplicated from the OEM. It may be an exact replica or may vary slightly in fit or dimensions. In almost all cases, cheaper labor and materials are used in production which lowers the retail cost. This does not mean inferior quality but it usually carries that reputation. An aftermarket part likely never has TUV status because it either would fail TUV testing or the manufacturer decides not to submit it. I generally consider aftermarket parts to be more trouble than the savings are worth.
Performance Aftermarket
A part that can come from anywhere but is usually re-engineered to be better than OEM. This manufacturer will never have OEM status because the part is not produced to the same specs as the original piece. Yet it is superior in fit or performance. Best example is Meyle HD products which improve on the OEM design. I wish there were more of these, especially from the OEMs themselves!
I believe credit for the original tiers in the BMW market goes to Vincent at BimmerZone and Doug at Turner Motorsport, with some additional input from me. FCP Euro has recently confused things with some re-definitions but I think my above list is the best representation of the current market.
Bottom line: ask your supplier for their opinion. But if they can't give you an honest answer then look elsewhere. Ask which brand has the highest return rate. If you show a little interest and trust his/her opinion you should get what you want. It's not all about the price.
"The bitterness of poor quality remains long after low pricing is forgotten," - Leon Cautillo, author.