Goodwill usually stays constant on the balance sheet, so why would it be impaired, and what does goodwill impairment mean?

• It is written down when the acquired business's value falls below carrying value; a non-cash charge signaling the deal underperformed
• It is written down every year on a fixed schedule, the same way a finite-lived intangible such as a patent is amortized evenly over its useful life
• Impairment means goodwill has gained value, so the company marks it up to reflect the acquisition's improved outlook
• It is a cash payment made to the seller after closing, recorded when an earnout or holdback finally comes due
• It can only occur when the company itself is sold, at which point goodwill is trued up to the new deal's price