Let $E_j$ be a D11: Set for every $j \in J$.
The union of $E = \{ E_j \}_{j \in J}$ is the D11: Set \begin{equation} \bigcup_{j \in J} E_j : = \{ x \mid \exists \, j \in J : x \in E_j \} \end{equation}

Let $x$ be a D11: Set.
The union of $x$ is the D11: Set \begin{equation} \cup x : = \{ z \mid \exists \, y \in x : z \in y \} \end{equation}