A reservation in international law is a caveat to a state's acceptance of a treaty. A reservation is defined by the 1969 Vienna Convention on the Law of Treaties (VCLT) as: a unilateral statement, however phrased or named, made by a State, when signing, ratifying, accepting, approving or acceding to a treaty, whereby it purports to exclude or to modify the legal effect of certain provisions of the treaty in their application to that State. (Article 2 (1)(d))
In effect, a reservation allows the state to be a party to the treaty, while excluding the legal effect of that specific provision in the treaty to which it objects. States cannot take reservations after they have accepted the treaty; a reservation must be made at the time that the treaty affects the State. The Vienna Convention did not create the concept of reservations but codified existing customary law. Thus even States that have not formally acceded to the Vienna Convention act as if they had. As reservations are defined under the Vienna Convention and interpretative declarations are not, the two are sometimes difficult to discern from each other. Unlike a reservation, a declaration is not meant to affect the State's legal obligations but is attached to State's consent to a treaty to explain or interpret what the State deems unclear.
Procedure
The Articles 19–23 of the Vienna Convention details the procedures relating to reservations. To see if a reservation is valid the legality of the reservation test applies as described in article 19 of the Vienna Convention. According to this article a state may not formulate a reservation if:
1: The reservation is prohibited by the treaty. (e.g. the Supplementary Convention on the Abolition of Slavery and Convention against Discrimination in Education) 2: The treaty provides that only specified reservations, which do not include the reservation in question, may be made. This is often the case when during negotiations it becomes apparent that a certain provision in a treaty will not be agreed upon by all parties. Therefore, the possibility is given to parties not to agree with that provision but to agree with the treaty in general. 3: In cases not falling under (1) or (2), the reservation is incompatible with the object and purpose of the treaty. Point 3 is called the compatibility test and is difficult to determine. It is not always clear what the object and purpose of the treaty is, especially when treaties are long and complex.
A reservation must be put into writing and then sent to either the depository of the treaty, in the case of a multilateral treaty, or directly to the other States party to the treaty.
A State may withdraw a reservation at any time. This requires written submission to the other signatory States.
Objections
When states make an objection to a reservation stating that it failed the legality test, there are three possible results, according to legal commentators:
The state which made the reservation is not bound by the treaty anymore. In other words, the state is no longer a party to this treaty. The state which made the reservation is bound by the treaty including the parts on which it made the reservation. The state which made the reservation is bound by the treaty, but not by the part on which it made the reservation.
There are many opponents to the second option who argue that this goes against the principle of state consent. States can only be bound by provisions they have consented to. Since they have made a reservation to a certain provision, they cannot be bound by it.
According to some commentators, what happens in practice in the VCLT regime is the third option. What follows from article 20 paragraph 4(b) and article 21 paragraph 3 VCLT is that the only thing which can happen is, that if an objecting state feels very strongly about a reservation, it will state that the whole treaty is not in effect between the reserving state and itself. This rarely happens, thus the reservation stands, whether it passed the legality test or not.
Others think differently on this. According to Anthony Aust, “if one or more contracting states have objected to the reservation as being prohibited, the reserving state must decide whether or not it is prepared to be a party without the reservation; and until it has made its position clear it cannot be regarded as a party”.
The difference of opinion on this is the fact that it is not likely that the articles 20 and 21 apply to reservations, which cannot to be made according to article 19 of the VCLT. However, they are applied in practice.