Time Machine in macOS can automatically back up every volume you have mounted on a Mac except those that you exclude. You manage the exclusions list by opening the Time Machine preference pane and clicking Options, where you can also add particular folder hierarchies, which appears as paths. (To exclude your Documents folder, for instance, you click the + sign and navigate to it to select the folder, but it appears in the list as ~/Documents, where the tilde means “your home directory.”)
You may notice a few items are always grayed out. These are volumes identified as Time Machine backups, and typically display the Time Machine icon as their volume icon. Apple doesn’t let Time Machine back up Time Machine volumes. You can use third-party cloning or incremental backup software—like Carbon Copy Cloner, ChronoSync, or SuperDuper—to create a separate backup of the backup.
It might seem like a paradox that an item is listed in the exclusion list that you can’t exclude. However, Apple likely decided it would be more confusing to exclude displaying those volumes, because then you might wonder why they aren’t showing up at all.
Read more at MacWorld.com