French to English Other Government / Politics Diplomatic terminology

In a caption to a photo on a Press Release ... "lors de la signature des alternats portant établissement des relations diplomatiques."

The Press Release refers to a signing ceremony for an agreement establishing diplomatic relations.

I have never come across "alternats" before. I can only find one reference to the English word "alternat" in a dictionary, as a noun referring to a diplomatic term used for signing treaties, etc., in a specific order according to rank.

Can one say ... "signing the alternats establishing diplomatic relations?" I'm not sure how this word should be used.

All suggestions appreciated!