To quote from "Cisco IOS Releases - The Complete Reference":
'fc2' is an internal rebuild of the 'fc1' build. 'fc2' is usually created to fix a specific defect found in the 'fc1' build. An 'fc2' build cannot be created if the first build was already released to a public directory.
I'm not exactly sure what that means in practical terms but it sounds like fc2 is a last-minute bugfix.
The actual bug (or "caveat" in Cisco-speak) should be detailed in the release notes, but it won't say which one caused the rebuild, although in all likelihood it's the one at the bottom of the table.
Cisco IOS images usually contain fc1 or fc2 designators following the "release type" field. When the software has not been released to the public and Cisco testing reveals a major defect, an fc2 rebuild occurs. The rebuild number of the images remains exactly the same as the non-released fc1.
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