FCPXML format
FCPXML is a specialized format that uses XML elements to describe the data passing between Final Cut Pro and other applications.
How Media Sync Tool uses it
Media Sync Tool reads and writes FCPXML to exchange clips with Final Cut Pro. It opens the FCPXML you export from your event, builds the synced and multicam clips, and writes the result back out as FCPXML for you to import into Final Cut Pro.
Learn more
To learn more about FCPXML, visit the Final Cut Pro User Guide.