For the 80% limitation under §172: when a corporation uses post-2017 NOLs (losses arising in tax years beginning after December 31, 2017) to offset taxable income, the deduction is capped at 80% of taxable income (computed without the NOL deduction, section 199A deduction, or section 250 deduction), minus any pre-2018 NOL carryovers applied to that year. This means post-2017 NOLs can shelter at most 80% of the corporation's current-year taxable income, leaving at least 20% subject to tax. Pre-2018 NOLs are applied first and are not subject to the 80% cap.

Exception: Insurance companies (other than life insurance companies) are exempt from the 80% limitation — their NOLs can fully offset taxable income.

Waiving the carryback period: If a corporation has a farming loss or qualifies as an insurance company and is eligible for the 2-year carryback, it may elect to waive the carryback and carry the NOL forward instead. The election must be made by the return's due date (including extensions) and is generally irrevocable.