To access the Trailers tab, the Trailers service should be enabled in the account properties. To work with trailers and their groups, the user should have the Create, edit, and delete trailers access right to the resource of the account.
In Wialon, trailers refer to additional equipment that is attached to the main vehicle and doesn't have its own tracker or controller. Monitoring trailers is similar to tracking drivers in many ways.
Video: Drivers, trailers, passengers | Basic
You can find information about the Trailers tab in the following video dated 02.06.2021: