For example, you can design digital products, like animal clipart or a downloadable wedding poem that could be printed by the customer. Your design could be resold thousands of times without needing to create each item or ship it. Once you create your digital product and list it on a site like Etsy or Ebay, the revenue flows in with little oversight.
As interest rates have been going down over the past 30 years, bond prices have continued to go up. With the 10-year yield (risk free rate) at roughly 2.55%, and the Fed Funds rate at 1.5% (two more 0.25% hikes are expected in 2018), it’s hard to see interest rates declining much further. That said, long term interest rates can stay low for a long time. Just look at Japanese interest rates, which are negative (inflation is higher than nominal interest rate).

The first application of residual income, the remaining money after debts are paid each month, is relevant when analyzing a person's financial status or ability to qualify for financing. The second application, the more widely recognized meaning of residual income, is money that is received on an on-going basis for work that is completed once. This form of income allows the recipient to generate revenue that is not based on time limits. Residual income is the foundation for wealth because it offers flexibility in earning and maximizing income. It also allows income to be generated long after the work has been done.
Residual income is calculated as net income less a charge for the cost of capital. The charge is known as the equity charge and is calculated as the value of equity capital multiplied by the cost of equity or the required rate of return on equity. Given the opportunity cost of equity, a company can have positive net income but negative residual income.