Reduction vs write off outstanding export

I can guess and in correlation with GDP formula C+I+G-X. RBI don't have money and always in a deficit but not literally. 

Reduction in invoice value= attracting debtors to pay my due 

Write off- since international trade is governments headache they write off your debts when a court can't convict them or too expensive to file against foreign traders. This is a gist and a wholesome one

My simple query is as follows 

In case of outstanding export realusation beyond 9 months, exporter has option either to write off as per status holder certificate or reduce invoice value

These are available with any AD dealing export bills.

Query is difference between reduction of invoice value and write off.

Only foreign trade and banking persons can throw light more effectively.

For quick understanding -

If invoice value is 100 and due to certain situation (like fluctuation in prices) the value of invoice is reduced say 90 , then exporter goes for invoice reduction . This comes into picture after a bill (set of export documents) has been negotiated or sent for collection to overseas bank.

If invoice value is 100 and exporter is unable to get the export proceeds , then he will go for write off . 

There are % criterias upto which write off and invoice reduction are applicable . pls refer latest master direction on export of goods & services for more info.