I have Volkswagen Golf or similar VAG model and it has a choice of 50mm or 55mm front suspension. What does this mean and how can I check which one I have?

This means the diameter of front damper where it connects to axle. These models have been manufactured with two different diameters and best option is to measure it. It is the same with all the models that have different diameters depending on the exact model of the car.