A rule of law is said to be "opposable" when it can be legally imposed on the person/entity to whom/which it is addressed. In international law, opposability of international texts is limited by the sovereign status of a State. On the one hand, a rule cannot be imposed on a State which is a sovereign entity (cf. Sovereignty, Conditions of a State's Commitment) unless it has previously agreed to comply with it. On the other hand, to compensate for this limitation, there are numerous legal texts which do not have a binding character and are not therefore opposable, but which contribute to the formulation and development of the law.
In fact, no international text adopted by States constitutes a binding rule of law for them, and is not therefore opposable to them. The binding aspect of a rule of law must be specified in the course of its formulation. Many international texts, such as resolutions passed by the United Nations General Asssembly, are not binding and are not opposable to the States as such. All of these texts are referred to as "soft law", as opposed to "hard law" (conventional and customary), to the extent that they contribute, by regularly reaffirming a certain number of principles, to the construction of international law (cf. Conventional Law / Customary Law).
The provisions of an international agreement are thus only opposable to the States which are party to them, but not to Other States. However, the content of the provisions laid down by an international agreement may be opposable to Other States if they concern rules of law which such States have accepted as part of customary law (the USA is not a party to the UNCLOS, but has recognized the principle of the Exclusive Economic Zone (EEZ)). The source of opposability here resides in the "custom" rather than in the international agreement, which is not opposable as such to Other States.
This principle is also valid for rules of law which arise from the functioning of an organization set up through an international agreement: they are opposable (on condition that the Parties have given them a binding character). However, some organizations, such as the General Fisheries Commission for the Mediterranean (GFCM) or the International Commission for the Conservation of Atlantic Tunas (ICCAT ) may adopt texts which do not dispose of this binding character and thus belong to "soft law"; they are therefore not opposable.