Reasons for disassociation of invoice to sales order:
1. Customer name was changed in the Invoice record
2. All line Items pre-populated from the Sales order are deleted
---Manually re-entering same items that are present on the Sales order still deletes the link to Sales Order
---The form will show> Created From: Sales Order #SOxxxx before saving but after saving the form, the value of Created From field will be deleted
---A new Revenue Arrangement will be created. In effect, there will be a Revenue Arrangement for both Sales Order and Invoice.
3. Project field of all line Items that were pre-populated from the Sales Order are changed on the Invoice