In this situation the user ID is the Box identifier for a user. User IDs can
found for any user via the GET /users endpoint, which is only available to
admins, or by calling the GET /users/me endpoint with an authenticated user
Please note that some of our SDKs create new clients for the other user, while
others modify the existing client and provide a way to return to a state where
the client authenticates for the original user itself.