What dictates the actual frequency of ESA astronaut flights to the ISS, rather than a set European quota?

Complex seat exchange negotiations between the agencies

The regularity with which European astronauts secure slots for flights to the International Space Station is not determined by a predefined, fixed quota allocated specifically to Europe. Instead, the actual frequency is a dynamic outcome resulting from intricate seat exchange negotiations occurring between the primary partner agencies. These agreements might involve ESA providing valuable support, such as supplying necessary ground support or specific hardware, in direct exchange for guaranteed astronaut flight opportunities aboard vehicles operated by partners like NASA or Russia. This dependency on negotiation means flight timing aligns with the current mission manifest and vehicle availability.