OEM (Original Equipment Manufacturer)
Brand that fits the car at the factory. Varta supplies many BMW/Audi/Mercedes; Tudor/Exide supplies Renault/PSA.
OEM (Original Equipment Manufacturer) is the brand that manufactures the component the car is fitted with at the factory. In batteries, the best-known relationships in Europe: Varta (Clarios group) factory-supplies many BMW, Audi, Mercedes and Volkswagen models; Tudor / Exide is the usual OEM for Renault, PSA (Peugeot/Citroën) and Dacia; Bosch supplies several European and Asian models; Banner has OEM contracts with the VAG group and commercial vehicles. OEM relationships change over time and by model, so it is always worth checking the original part-number for your vehicle if you want to exactly match the factory battery.
A battery OEM-identical to the original usually costs the same as or slightly more than an "aftermarket" equivalent (Yuasa, Berga, Bosch S4, Banner Energy Bull), but guarantees exact electrical compatibility. Equivalent-quality aftermarket is a perfectly valid replacement — what matters are the specifications (Ah, CCA, technology, dimensions, polarity), not the brand. The OEM part-number is printed on the original battery case; the quickest way to find a compatible equivalent is to search by registration plate, which already identifies the correct technology, capacity and dimensions.