Treaty
The contracting parties' full names or sovereign titles are often included in the preamble, along with the full names and titles of their representatives, and a boilerplate clause about how their representatives have communicated (or exchanged) their full powers (i.e., the official documents appointing them to act on behalf of their respective states) and found them in good or proper form.

Q: What are the official documents appointing a party's representative to act on their behalf?
full powers