Reservations are essentially caveats to a state's acceptance of a treaty. Reservations are unilateral statements purporting to <answer>exclude or to modify</answer> the legal obligation and its effects on the reserving state. These must be included at the time of signing or ratification, i.e. "a party cannot add a reservation after it has already joined a treaty".
A reserving party to a treaty may include a statement that attempts to do what to its legal obligations or their effects?
exclude or to modify