Personal Backup automatically excludes some files when an application marks them to be excluded from backups. These types of files are usually cache files, or files that would be recreated automatically by the software.
The exact list of files are commanded by the system. This is controlled by the extended attribute: "com.apple.metadata:com_apple_backup_excludeItem"
Personal Backup has also added some additional files to this list, including:
- .Spotlight-V100 (Spotlight database)
- .fseventsd (FSEvents database)
- /private/var/vm (virtual memory)
Applications may choose to exclude files from a backup because they're not required. This allows Personal Backup to run faster, reduce backup file sizes, and even improve reliability in some cases.