Some messages are geographic. A campaign wants to be seen on a particular main street, outside a particular event, during the particular two hours the people it is talking to are there. Fixed outdoor media cannot do that; a truck can.
DSD runs LED trucks with daylight-rated screens and on-board power, driven to a route and parked to a schedule. The message rotates, so a single trip carries three or four propositions rather than one.
Same content platform as the fixed estate, so the run is planned the same way a screen campaign is.