A more thorough explanation:

Plenary

Plenary means full, absolute, and unqualified. When someone has plenary authority over a project, they have complete power over that project. Similarly, a plenary session is a meeting called by group leaders with the intention of having the full group in attendance.

• A CEO has plenary authority over their company, meaning they have complete control over all decisions made.
• The United Nations holds plenary sessions where all member countries are expected to attend and participate in discussions.

The examples illustrate the definition of plenary by showing how someone or something has full and absolute power or participation. In the first example, the CEO has complete control over their company, while in the second example, all member countries are expected to attend and participate in the plenary session, leaving no room for partial attendance or participation.