Remark 9.3.4.19. Let $\operatorname{\mathcal{C}}$ be an $\infty $-category which admits pullbacks. Stated more informally, Remark 9.3.4.18 asserts that a morphism $f: X_0 \rightarrow X$ of $\operatorname{\mathcal{C}}$ is a monomorphism if and only if the relative diagonal $\delta _{X_0/X}: X_0 \rightarrow X_0 \times _{X} X_0$ is an isomorphism.
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