This is difficult question, even for seasoned diplomats! Recently, this question has been made only more complicated by the terminology proposed by celebrated publicist Anthony Aust, author of Modern Treaty Law and Practice. However, it is possible to explain the situation as follows: We are dealing here with “paper” between States. Let us refer to these documents or papers as “instruments” (not “agreements”). Some of these instruments are meant to create rights and obligations under international law; while others are only meant to spell out intentions and decisions that are political in nature but not legally binding. The first kind of document is in fact called “treaty” under the terms of the Vienna Convention on the Law of Treaties of 1969:
“treaty” means an international agreement concluded between States in written form and governed by international law, whether embodied in a single instrument or in two or more related instruments and whatever its particular designation
This means that an exchange of letters between two ministers of foreign affairs could very be a treaty, but how can one be sure? The only way is to look at the “actual words” of the instrument, because there are certain words that clearly indicate rights and obligations under international law (for instance “force” or “agree”), whereas other word convey the impression that the two States are trying to avoid legally binding language and instead use alternative terms such as “decide” (instead of “agree”).
Because what matters is the words used in the document itself, it is important to notice that the title (designation) of the instrument is irrelevant: a “charter” may well be a non-binding instrument, whereas a “Term of Reference” may well be a treaty.
In the past and even today, it was common for treaties to be drafted as instruments labeled “Memorandum of Understanding.” This was especially common among Commonwealth Member States. This is consistent with the fact that the designation is not relevant to the legal status of the instrument. However, it should be understood that some treaties are processed “in simplified form” whereas others are meant to deal with more significant or controversial topics, and are processed in “solemn form.” For instance, the law regulating treaties in a recently created State under UN guidance, Timor-Leste, explains “Treaties shall be in solemn form or in simplified form.”
In practice, an instrument labeled “Memorandum of Understanding” would typically be a treaty in simplified form, if it was meant to be a treaty rather than a non-binding instrument. Here, it is important to notice that other designations (“charter” and “accord” for instance) have been used for non-treaty documents.
Today, under the influence of UK guidance (following a controversy between the UK and the USA on the legal status of a particular “Memorandum of Understanding” (MOU), there is an increasing tendency to use the MOU label for non-binding (non-treaty) instruments. As a result, the legal status of MOU instruments has become unclear and confusing.
Adding to the confusion is the fact that noted specialist Anthony Aust (author of a major textbook in treaty law) has proposed the Treaty vs MOU dichotomy as a easy way to categorize intergovernmental “papers.” The problem is that many MOUs (“Memorandum of Understanding” instruments) are in fact treaties, and that some instruments designated as “charter” or “accord” are in fact MOUs to use Aust’s system.
In order to bring clarity to international practice and to assist diplomats, EUCLID has formulated the following categorization:
Binding and Registerable Instrument (“treaty”)
NBI (“MOU” in AUST)
‘accept’ ‘approve’ ‘decide’
‘agreement’ or ‘undertaking’
‘arrangement’ or ‘understanding’
‘authoritative’ or ‘authentic’
‘continue in force’
‘continue to have effect’
‘enter into force’
‘come into operation’ or ‘come into effect’
‘Governments’ or ‘Participants’
‘rights’, ‘have the right’
‘benefits’, ‘be permitted to’
‘shall’ ‘undertake to’ ‘agree to’ or ‘undertake’
‘will’ or ‘decide’
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