Other forums have discussed this in the past, the order type is just that,
STOCK is for dealer stock
RETAIL is a customer special order
FLEET is for a customer w/ Fleet status
BASIC is basic
BASIC OFF STANDARD EQUIPMENT usually seen on vehicles such as C/S, BOSS etc. cars having special parts etc.
FoMoCo used these order types to group vehicles on the assembly line or to prioritize assembly. For example RETAIL orders were cars which for all intents and purposes had been sold so it was a higher priority than a STOCK order.
I am assuming; based on where in the vehicle order image the information appears the DSO ITEM # is the number assigned to the vehicle for the Dealer, District Sales Office or factory to track the order. I have not seen much discussion on the DSO ITEM #.