Nature of business is similar to occupation, but it's not always uniformed. As an example, a customer might have the occupation Shop Keeper, but the nature of business will likely differ customer to customer. As an example below -
Shop Keep is the occupation, employers business could be any of the following and more.
Grocer
Butchers
Off-licence
Supermarket
Electronics
It is important to establish this fact and will give you and the underwriters a better understanding to what the van is being used for.