An amended tax return corrects a previously filed tax return, used when new information surfaces or an error is discovered after the original return was already submitted.
A common startup scenario: a company files its return, then later determines it actually qualified for the R&D tax credit or missed a deduction, prompting an amended filing to claim the correction. Amended returns can also work in the company's favor retroactively, part of why an unclaimed prior-year credit isn't necessarily gone just because the original deadline passed.
There are real time limits on how far back an amendment can reach, generally a few years from the original filing or payment date, so an unclaimed credit or missed deduction is worth chasing down before that window closes rather than assuming it's always available to fix later.
Add your revenue and website for a full diligence brief, reviewed by a CPA who ran EY's West Coast R&D Tax Credit practice for 13 years.