What is the difference between Ignore and Mark as Fixed for broken links?

When you view a list of broken links such as the list of unique broken links, you can click the gear icon and choose either Ignore or Mark as fixed. This document provides an explanation for how each option works:

• Ignore indicates to Acquia Optimize that you don't want to see an error for this link ever again. When this option is selected, the link disappears from the list of broken links and will not return even after the next scan. Use this option if links are marked as dead when really they are not. You can also choose to ignore spelling issues on a single page basis, and all other issues on a domain level. Ignoring issues will result in a recalculated compliance score, as the system will regard these ‘ignored’ issues as being non-existent. If you ignore a flagged issue, it is not factored into future scans, and this will permanently affect the compliance score.
• Mark as fixed is used when you have actually fixed the problem on your website HTML and want to remove the link from the lists. Do this to "check off" that link so you have a better overview of the next tasks you need to complete. If we find this dead link on subsequent scans it will be back in the list and factored into the compliance score.

The information from this document is included in the Acquia Optimize documents:

Quality Assurance: Links

Quick Guide: Fix Broken Links