The figure is calculated from components reported in SEC filings rather than tagged as a single element. The starting point was historically tagged IncomeLossBeforeExtraordinaryItemsAndCumulativeEffectOfChangeInAccountingPrinciple, an element the taxonomy has since deprecated. The SEC's documentation for it describes income from continuing operations after tax plus the results of discontinued operations, but before extraordinary items. From that, the noncontrolling share, tagged NetIncomeLossAttributableToNoncontrollingInterest, and preferred dividends, tagged PreferredStockDividendsIncomeStatementImpact, are subtracted.
Extraordinary items were removed from US GAAP by ASU 2015-01, effective for fiscal years beginning after December 15, 2015, and cumulative-effect accounting changes were largely replaced by retrospective restatement under ASC 250. For recent periods, therefore, this measure normally equals net income available to common shareholders. It differs mainly in older periods, or where a company makes adjustments to common earnings other than preferred dividends, such as earnings allocated to participating securities under the two-class method.
The measure is most useful in long historical series. It gives a consistent common-shareholder earnings figure across periods before and after the rule changes, so growth rates and price-to-earnings histories are not distorted by one-time extraordinary gains or losses in older years. Note that under the SEC's element definition it still includes discontinued operations, so it is broader than income from continuing operations.