Remark 5.2.5.6 (Base Change). Suppose we are given a pullback diagram of simplicial sets
\[ \xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{E}}' \ar [r] \ar [d]^{U'} & \operatorname{\mathcal{E}}\ar [d]^{U} \\ \operatorname{\mathcal{C}}' \ar [r] & \operatorname{\mathcal{C}}, } \]
where $U$ and $U'$ are cocartesian fibrations. Then the homotopy transport representation $\operatorname{hTr}_{\operatorname{\mathcal{E}}'/\operatorname{\mathcal{C}}'}$ is isomorphic to the composite functor
\[ \mathrm{h} \mathit{\operatorname{\mathcal{C}}'} \rightarrow \mathrm{h} \mathit{\operatorname{\mathcal{C}}} \xrightarrow { \operatorname{hTr}_{ \operatorname{\mathcal{E}}/\operatorname{\mathcal{C}}} } \mathrm{h} \mathit{\operatorname{QCat}}. \]