What public conventions are

[P]ublic conventions are such as can be made only by the right of a higher or lower authority of government; and in this respect they differ not only from the contracts of private persons, but also from the contracts of kings which are concerned with private affairs. However, from such private contracts…causes of war are wont to arise, although more frequently from public contracts. Having, therefore, sufficiently treated of compacts in general, we ought to add some details which relate to this more excellent kind of agreement.

Division of conventions into treaties, sponsions, and other agreements

We can divide these public conventions, which the Greeks call συνθήκας (articles of agreement), into treaties, sponsions, and other agreements.

The difference between treaties and sponsions

[T]reaties are made by order of the highest authority; and… in respect to such treaties, the people itself is liable to divine wrath if it does not keep its agreements. … A sponsion is made when those who do not have from the supreme authority a commission for such an act promise something which essentially affects that authority.