Let $X \neq \emptyset$ be a D11: Set.
Let $R \subseteq X \times Y$ be a D4: Binary relation.
The relation class of $x \in X$ with respect to $R$ is the D11: Set \begin{equation} R(x) : = \{ y \mid (x, y) \in R \} \end{equation}

Let $B = (X \times Y, R)$ be a D4: Binary relation such that

(i)	$X \neq \emptyset$

The relation class of $x \in X$ in $B$ is the D11: Set \begin{equation} R(x) : = \{ y : (x, y) \in R \} \end{equation}