The last vehicle the customer had searched on will be saved and displayed on the order view page, allowing whoever fulfills the order to verify that the customer purchased a part for the correct application fitment.
This affords your company an additional chance to catch and correct erroneous catalog information before the shipment is executed, ultimately reducing incorrect orders.
You have the ability to force the customer to select their vehicle before they can proceed with checkout. If enabled, it will automatically do it for products that have vehicles mapped to them. Upon selecting add to cart, if they do not already have a vehicle selected in their session, but there are vehicles available for the selected product, they will be redirected to the intermediate page.
On the intermediate page the customer is presented with a chained select-box interface exactly like the main vehicle search, but only populated with the vehicles for the selected product. The customer must enter in the vehicle before proceeding with checkout. This ensures the customer has a chance to verify compatability. This feature is completely optional and if enabled will only appear where it makes sense.