Hi Rajesh,
Regarding the Rate and Unit Price fields, I checked the DEMO database and found it as follows :
1) Rate field in document : this field talks about the Exchange Rate for the day on which the document is being entered. So, if you create a All currencies vendor and give a Exchange Rate for say Euro as 1.25, the same will appear in the rate field in the rows.
2) Unit Price : is the price field which needs to be entered for booking the value of the item in question.
It is applicable to PO as well.
Hope it clarifies your second question.
Regards,
Jitin
SAP Business One Forum Team