Let $X_j$ be a D11: Set for each $j \in J$.
The disjoint union of $X = \{ X_j \}_{j \in J}$ is the D11: Set \begin{equation} \coprod_{j \in J} X_j : = \bigcup_{j \in J} \{ (x, j) :x \in X_j \} \end{equation}