= Solution
Let $U_1\subseteq U_2$ be transitive classes, both interpreted with actual membership. A formula $\phi(\bar x)$ is upward absolute from $U_1$ to $U_2$ if for every tuple of parameters $\bar a$ from $U_1$,
$$
\boxed{(U_1,\in)\models\phi(\bar a)\quad\Longrightarrow\quad(U_2,\in)\models\phi(\bar a).}
$$
For proper classes this means the usual relativization of the fixed formula's <quantifiers>. Full absoluteness requires the implication in both directions. The qualification on parameters matters: the smaller class must contain every object named in the comparison.
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