Sales Order Type are nothing but the Transaction Type field which we see on Sales Order Headers.
Majorly there are 3 types of Sales Orders, they are:
- Standard Sales Order
- Mixed Sales Order
- Return Sales Order
In Standard Sales Order we have
- Back to Back
- Drop Shipment
Internal Sales Order are raised within an organization
You can query for OE_TRANSACTION_TYPES_TL table for complete list of Sales Order types specific to your instance.