Arcade ROMs are essentially dumps of multiple chips on a physical circuit board. As emulation improves, developers find better, cleaner dumps of these chips. When the FBNeo team updates their emulator to support a more "perfect" dump, the old files in your .zip folder no longer match the expected .
When FBNeo says "unknown," it’s telling you: "I see a zip file here, but the data inside doesn't match the specific version I'm programmed to read." Step 1: Matching Versions (The Golden Rule) romset+is+unknown+fbneo+fixed+free
Go to the official FBNeo GitHub or website and download the .dat file for the latest release. This file is the "blueprint" that tells the manager exactly what each ROM should look like. Arcade ROMs are essentially dumps of multiple chips
Check out donal on bbc radio & aseem on sky tv talking "the big fat fix"