After xray dr said ulna had pulled away from distal radius he put arm in sling and got to go back in 7 days. what does this mean and what is treatment?

You need a specialis: If your distal ulna pulled away from the radius you have some structure caught in between the bones. This needs surgery to reduce and then you need to be in a cast not a sling.

DRUJ injury: There is a joint in the wrist called the distal radioulnar joint (druj). This joint allows the forearm to rotate and if it becomes subluxed or dislocated, pain and difficulty with motion occurs. Typically it can be treated by immobilizing the wrist in supination (palm facing the sky). Sometimes it requires surgery to reduce and stabilize the bone or repair cartilage. See a hand surgeon.