The Jump_OK flag is used in several ways. Unprivileged programs cannot use MOVETO on an object unless the player either controls the object, the room it's being moved from, and the room it's being moved to, or else they are set Jump_OK. A player cannot use an action that is linked to another player unless the other player is set Jump_OK. On some systems, where SECURE_TELEPORTing is set up, you cannot use an action to leave a room, unless the action is either attached to that room, or the room is JUMP_OK.