The production of F Form is mandatory for all the transfer to principal and his agent. In case the Form is not produced along with the LR, Delivery Challan / Stock Transfer Memo etc the transaction will be considered as a "sale" as per Section 6-A (2) of CST Act.
As mentioned above the Assessing officer is free to examine whether the transaction is genuine "otherwise than by way of a sale" i.e stock transfer to his agent etc even after furnishing the Form F.