Effective Mating

It is the distance from when the Mating contacts start to make contact until it reaches the specified Mating state when mating.
At this distance, foreign matter is removed by a phenomenon called "wiping", and in addition, the long effective Mating length, which is advantageous against misalignment, etc., leads to high reliability.
On the other hand, an excessively long effective Mating length unnecessarily increases the size of the connector and tends to result in a stub shape/state that hinders high-speed transmission.