When payments are settled on a net basis, the parties to the payments offset the amounts they are due to pay and receive with each other (or with a central party, or clearinghouse) and maintain a running balance of the netted amounts.
If the debts are not offset or netted, however, the taxpayer could pay a whopping 4-1/2 percent differential on the interest it pays on the $100 it owes for 1986 as compared with the interest it receives on the $100 the Government owes the taxpayer for 1985 even though there is no true net debt (as of March 15, 1987) owed to the Government.