The U.S Treasury statement for August 2015 reports the following:

Receipts:

Individual Income Tax $ 103 B

Social Security & Other Payroll Taxes 83 B

Corporate Income Tax 2 B

Other Taxes, etc. 23 B

Outlays by Function:

Defense $ 40 B

Social Security 75 B

Medicare 24 B

Interest on Debt 25 B

Other 111 B

Calculate (i) the total surplus (or deficit) and (ii) the primary surplus (or deficit).

What is the logical Reason for distinguishing between the two measures?