The DEX Manager Facet is used to approve/unapprove DEX addresses for use in the LibSwap library. It reads and writes from the global storage defined in LibStorage.
Facets that use swapping inherit from the Swapper.sol contract which checks the whitelist before making any swaps.
The DEX Manager manages which contracts and functions can be executed through the LI.FI main contract. This can be updated by a single admin key which if compromised could lead to malicious code being added to the allow list.