A base plate is the license plate a truck carries from its state of registration — its “base” state. For commercial vehicles that run across state lines, that plate ties into apportioned registration, where fees are shared among the states the truck operates in based on miles driven.
Along with the cab card, the base plate is part of proving a truck is properly registered to operate interstate. Dispatchers and drivers keep this paperwork current because expired or mismatched registration can sideline a truck at inspection.