Open Pitlane mode is all-day free driving without dividing into groups.
Driving in groups A and B (always a maximum of two groups) is required on circuits with low capacity.
The divisding into groups takes place according to the car and the level of experience. Thanks to this, the groups are homogeneous and you will enjoy your ride more. When group riding, each group usually has 8x 30 min. driving, which is more than enough for a serial or modified car and for your concentration.