From manual;
c_move(ENTITY* entity,VECTOR* reldist,VECTOR* absdist,var mode)
The first vector reldist gives a relative distance and direction in rotated entity coordinates, i.e. the direction that the entity is facing. The second vector absdist gives an absolute distance and direction in world coordinates. The resulting movement is a combination of both distances. Normally the first vector is used for the propelling speed of the entity, and the second one for external forces, like gravity and drift. To zero one of these vectors, the predefined nullvector can be given.