Object oA moves from it's current location to vX,vY by STEPSIZE pixels every step. fDONEFLAG is reset when the command is issued, and set when the object reaches it's destination. If STEPSIZE is 0, the current step size of object oA is used. If object oA is ego (object 0), the interpreter executes the program.control command.
Warning: If your program is relying on the flag fDONEFLAG to be set before continuing with something, such as going to the next part of a cutscene, make sure that the object will always be able to reach it's destination. It is possible that it could get caught behind a barrier or something, and not be able to get where it needs to go, in which case the game might sit there forever doing nothing. This is more of a problem when the object being moved is ego - it might work fine from where you test it, but it may be possible for the player to move ego into such a position where they will get "stuck" when being moved.