If you download a verified file, here is exactly what you are unlocking:
He snorted. “We enhance them. People want better visuals, more immersion. Some want more story. Some want a place to be remembered.” nfs carbon redux save game extra quality
If you are downloading this to use, ensure your version of the NFS Carbon Redux mod matches the version the save was created for, or the cars may handle differently than the save file intends. Report: Need for Speed: Carbon Redux – Extra