Hi,
If you see the definition then it may clarify your doubt,
Definition:
The order category determines the technical properties of orders.
You need to assign each order type to an order category, which can have more than one order type.
Orders have different purposes in the R/3 System. The structure and function of an order, as well as the transactions you use to process it, all depend on the order category.
Regards,
Narresh 