Two things,
1. Why VOV6 has account Assignment Category?
Ans. Sometimes, in order to fulfill Sales Order we need to do external procurement (PR). Especially when we are not manufacturing it. We can differentiate between different Purchase process on the basis of the combination of Item Category and Account Assignment Category. Thats why VOV6 contains both Item Category and Account Assignment Category
2. What is the use of Account Assignment Category?
Ans. Every PO raised by an user is a cost to the company. If we raise a PO we should know who is accountable (which department) and hence the G/L account to which cost will be booked against. Account assignment category is used for this purpose. With this we can display fields in PO like Sales Order, Cost Center, Internal Order etc.