Let $\mathbb{B} = \{ 0, 1 \}$ be the D217: Set of boolean numbers.
A D18: Map $f : X \to Y$ is a boolean function if and only if \begin{equation} Y \subseteq \mathbb{B} \end{equation}

A D18: Map $f : X \to Y$ is a boolean function if and only if \begin{equation} Y \subseteq \{ 0, 1 \} \end{equation}