Eight BSD flags are available in APFS to lock files, hide them, and restrict them in other ways. None of them synced correctly in iCloud Drive, and four caused other problems.
BSD
How the kernel relies on its extensions to make your Mac work, and has highest privilege for stability and security. How this must change for Apple Silicon.
Being a hybrid, naming disks and volumes can get complex in OS X. This article demystifies them.
The Eclectic Light Company 