The tab Trade in contains basic information for Trade in Vehicle.
The user, can edit the trade-in information stored (If the action: 'Edit Trade-in' is selected), or to remove the trade-in from this lead by selecting the action 'Remove trade-in'. In order to add a trade in user selects from Action Button menu Actions>>Trade in>>Add trade in Vehicle.
For further information please refer to this page: Add TradeIn Vehicle