Judging by the rules(md).ini comment, this flag was meant to specify whether or not this vehicle should be drawn tilted when driving on slopes. However, while the flag is parsed, there is no function assigned to it. In other words, this flag doesn't do anything. Vehicles tilt on slopes as long as they use voxel graphics and use the drive or tunnel locomotor. Vehicles using drive locomotor can also be tilted by or warheads.