“[T]he stream of continuing payments that are earned by Brad and/or Karen from the commissions (current and future) earned by the brokers within a book of business. Residual income arises from all sources of income relating to or derived from an identified book of business, including commissions earned by the brokers within that identified book of business from any source whatsoever.”

Case Schiller only tracks price appreciation of RE. RE as rental investment vehicle is measured primarily on rental yield or cap rate or some other measure. Price appreciation in that scenario is only a secondary means of growth, and arguably should be ignored as a predictor of returns when deciding on whether or not to invest in rentals. More important key performance indicators for rentals are net operating income and cash ROI. Appreciation, if it occurs, is a bonus.
Companies use the return on investment, or ROI, ratio as a method to measure the rate of return of a company's capital investments. ROI gives companies a means to compare the effectiveness and profitability of any number of investments. Residual income measures the net income an investment earns beyond the lowest return on its operational assets. Each of these measurements enables company management to determine the profit potential of investments in capital improvement, proposed projects or potential acquisitions.
Residual income is the amount of net income generated in excess of the minimum rate of return. Residual income concepts have been used in a number of contexts, including as a measurement of internal corporate performance whereby a company's management team evaluates the return generated relative to the company's minimum required return. Alternatively, in personal finance, residual income is the level of income that an individual has after the deduction of all personal debts and expenses have been paid.