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Remark Let $F: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ be a functor of $\infty $-categories. Then:


To each object $X \in \operatorname{\mathcal{C}}$ the functor $F$ assigns an object of $\operatorname{\mathcal{D}}$, which we will denote by $F(X)$ (or sometimes more simply by $FX$).


To each morphism $f: X \rightarrow Y$ in the $\infty $-category $\operatorname{\mathcal{C}}$, the functor $F$ assigns a morphism $F(f): F(X) \rightarrow F(Y)$ in the $\infty $-category $\operatorname{\mathcal{D}}$.


For every object $X \in \operatorname{\mathcal{C}}$, the functor $F$ carries the identity morphism $\operatorname{id}_{X}: X \rightarrow X$ in $\operatorname{\mathcal{C}}$ to the identity morphism $\operatorname{id}_{F(X)}: F(X) \rightarrow F(X)$ in $\operatorname{\mathcal{D}}$.


If $f: X \rightarrow Y$ and $g: Y \rightarrow Z$ are morphisms in $\operatorname{\mathcal{C}}$ and $h$ is a composition of $f$ and $g$ (in the sense of Definition, then the morphism $F(h): F(X) \rightarrow F(Z)$ is a composition of $F(f)$ and $F(g)$.