The FASB removed the extraordinary item concept with ASU 2015-01, effective for fiscal years beginning after December 15, 2015. Before that, a gain or loss qualified only if it was both highly abnormal for the company and not reasonably expected to recur, judged against the company's industry and location. The bar was high and few items met it. Qualifying examples included some losses from major natural disasters in areas where they were rare and from government expropriation of assets.
In XBRL the amount was tagged ExtraordinaryItemNetOfTax, now deprecated, with per-share amounts tagged separately, such as IncomeLossFromExtraordinaryItemsNetOfTaxPerBasicShare. The figure was always reported net of its own income tax, which did not appear in the income tax expense line. Earnings per share before and after extraordinary items were both required.
For periods after the change, this field is normally zero. Events that once might have been extraordinary are now reported within continuing operations, typically as their own line or in the notes, and analysts adjust for them case by case. For older periods, the field shows how much of a year's net income came from events outside normal business. Excluding it gives a cleaner base for long-run earnings growth and historical valuation multiples. The sign convention makes it easy to add back to net income, since adding a loss that is shown as positive restores income before the item.