Let $M = (X, \mathcal{F}, \mu)$ be a D1158: Measure space.
The set of subnull sets with respect to $M$ is the D11: Set \begin{equation} \{ E \subseteq X \mid \exists \, F \in \mathcal{F} : E \subseteq F \text{ and } \mu(F) = 0 \} \end{equation}