Top load means placing a vehicle on the upper deck of a two-level open car carrier. On the lower deck, a car sits beneath other vehicles and closer to the road. On the top deck, nothing is above it, so it avoids fluid drips from cars overhead and some of the spray and debris kicked up from the pavement. It is still exposed to weather.
Shippers request top load for vehicles with new paint, delicate finishes, soft tops or high value. Some carriers offer it as an option, sometimes for an added fee, because top-deck spots are limited and carriers need flexibility to arrange the load by weight, height and delivery order.
There are limits. Heavier and taller vehicles, such as full-size pickups and large SUVs, may need to ride on the lower deck for weight and clearance reasons. An inoperable vehicle can also be harder to place up top, depending on the carrier's equipment. And a top-load request does not change the carrier's cargo insurance terms.
When exposure is a real concern, enclosed transport is the more complete answer. See open vs. enclosed car transport to compare the two.