Dear Sharmista,
Probably you are trying to understand the behavior of the system technically while FU is created.
If I am correct, you can go to transaction /BOBF/CONF_UI to see the design of TRQ and TOR object in order to find out all the determinations and actions involved which get's executed while creation of FU and OTR.
However SAP recommends to enable the Automatic freight unit creation for order type. Based on the Freight unit building rule and freight unit building condition, FU will be created.
Thanks,
Bharath.