Since the phrase "romset is unknown fbneo fixed" usually refers to troubleshooting an error in emulation (specifically when using the FinalBurn Neo emulator), I have interpreted your request as an essay explaining .
(a database) to verify games. This database contains "signatures" (CRC or SHA-1 hashes) for every file inside a game's ZIP folder. MAME Documentation The Conflict: romset is unknown fbneo fixed
emulator fails to recognize a game file because it doesn't match the expected database entry, but the issue has been resolved Since the phrase "romset is unknown fbneo fixed"
Use a DAT file and CLRMamePro to verify the integrity of your files. MAME Documentation The Conflict: emulator fails to recognize
When in doubt, use "Non-Merged" sets to avoid dependency issues.
If you are using RetroArch with the FB Alpha 2012 or FB Neo core, note that RetroArch sometimes lags behind the standalone FBNeo. However, the "romset is unknown" error still applies.
Many popular arcade games will not boot without system-level files. If FBNeo looks inside a game ZIP and cannot find the BIOS, it may flag the set as unknown.