MED stands for “Minimum Engagement Distance”. This is the closest you are permitted to be to another player and still be allowed to pull the trigger.
As the BB’s are quite light weight the decelerate quickly. By allowing a certain distance between the shooter and the target we allow time for the BB to lose energy so it hits the target with less force. The more powerful the gun, the greater the MED. This works well with gaming as high powered guns are favoured by snipers who are trying to hit targets further away anyway.