In an open economy, savings have two functions, one is domestic investment, and the other is foreign investment. The current account balance includes net export NX and net factor expenditure NFP (net interest income). Therefore, NFP generally ignores S=I+(NX+NFP) and chooses S-I=NX, that is, net investment equals net export.

So NFI=NX+NFP, and it is generally believed that NFI=NX.

Net income of foreign investment

Generally speaking, if a country is a net foreign creditor, its investment income should be positive, and if it is a net foreign debtor, its investment income should be negative.

As the world's largest net foreign creditor, Japan's investment income is positive all the year round, which is a typical situation. By the end of 20 17, Japan's foreign financial assets (i.e. stock of foreign investment) were 10 12.43 trillion yen, its liabilities (i.e. stock of foreign capital utilization or stock of foreign investment) were 683.98 trillion yen, and its net foreign creditor's rights were 328.45 trillion yen, equivalent to 69.3% of the current GDP.

In 20 17, Japan's investment income surplus was 2.06 trillion yen, of which the investment income income (that is, the return income of foreign investment) was 310.23 trillion yen, and the expenditure (that is, the cost of utilizing foreign capital) was 5.26 trillion yen.