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The Vienna Convention on the Law of Treaties (VCLT) is a treaty concerning the international law on treaties between states. (Wikipedia)

A treaty is an agreement under international law entered into by actors in international law, namely sovereign states and international organizations. A treaty may also be known as an (international) agreement, protocol, covenant, convention, pact, or exchange of letters, among other terms. Regardless of terminology, all of these forms of agreements are, under international law, equally considered treaties and the rules are the same. (Wikipedia)

Sources of international law include treaties, international customs, general principles of law as recognized around the world, the decisions of national and lower courts, and scholarly writings. (Wikipedia)

Self-determination denotes the legal right of people to decide their own destiny in the international order. Self-determination is a core principle of international law, arising from customary international law, but also recognized as a general principle of law, and enshrined in a number of international treaties. (law.cornell.edu)