This function can be used in two ways depending on what you wish to check. You can give it an object_index to check for, in which case this function will return true if any active instances of the specified object exist in the current room, or you can also supply it with an instance id, in which case this function will return true if that specific instance exists and is active in the current room.
Please note that this function does not take into account those instances that have been deactivated using the instance_deactivate functions.
Argument Description obj The object or instance to check for the existence of.
score += 200;
The above code checks to see if any instances of the object "obj_Enemy" exist and if not it adds to the variable "score" and changes room.