The term protocol is used for agreements less formal than those entitled 'treaty' or 'convention'. The term could be used to cover the following kinds of instruments:
- A Protocol of Signature is an instrument subsidiary to a treaty, and drawn up by the same parties. Such a Protocol deals with ancillary matters such as the interpretation of particular clauses of the treaty, those formal clauses not inserted in the treaty, or the regulation of technical matters. Ratification of the treaty will normally ipso facto involve ratification of such a Protocol.
- An Optional Protocol to a Treaty is an instrument that establishes additional rights and obligations to a treaty. It is usually adopted on the same day, but is of independent character and subject to independent ratification. Such protocols enable certain parties of the treaty to establish among themselves a framework of obligations which reach further than the general treaty and to which not all parties of the general treaty consent, creating a 'two-tier system'. The Optional Protocol to the International Covenant on Civil and Political Rights of 1966 is a well-known example.
- A Protocol based on a Framework Treaty is an instrument with specific substantive obligations that implements the general objectives of a previous framework or umbrella convention. Such protocols ensure a more simplified and accelerated treaty-making process and have been used particularly in the field of international environmental law. An example is the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer adopted on the basis of Arts.2 and 8 of the 1985 Vienna Convention for the Protection of the Ozone Layer.
- A Protocol to amend is an instrument that contains provisions that amend one or various former treaties, such as the Protocol of 1946 amending the Agreements, Conventions and Protocols on Narcotic Drugs.
- A Protocol as a supplementary treaty is an instrument which contains supplementary provisions to a previous treaty, e.g. the 1967 Protocol relating to the Status of Refugees to the 1951 Convention relating to the Status of Refugees.
- A Proces-Verbal is an instrument that contains a record of certain understandings arrived at by the contracting parties.