What is Alternat?

Legal Definition
The name of a usage among diplomatists by which the rank and places of different powers, who have the same rights and pretensions to precedence, are changed from time to time, either in a certain regular order, or one determined by lot. In drawing up treaties and conventions, for example, it is the usage of certain powers to alternate, both in the preamble and the signatures, so that each power occupies, in the copy intended to be delivered to it, the first place. Wheat. Intern. Law, pt. 2, c. 3, 4..
-- Bouviers Law Dictionary
Legal Definition
A usage among diplomatists ,by which the rank and places of different powers, who have the same right and pretensions to precedence, are changed from time to time, either in a certain regular order or one determined by lot. In drawing up treaties and conventions, for example, it is the usage of certain powers to alternate, both in the preamble and the signatures, so that each power occupies, in the copy intended to be delivered to it, the first place. Wheat. Int Law, ยง 157.
-- Black's Law Dictionary