Convention of parameter naming - undocumented options

GNU has long pushed for a combination of short options (-h) and long options (--help), which goes a long way to clearing up the syntactical mess of commands like tar and dd, with their legacy syntax going back to the stone age. But I recently learned that GNU's parted has a _third_ kind of option, where an 'undocumented feature' can be accessed with ---pretend-input-tty. It looks like it was introduced in this commit in 2007. It's the first time I've seen that, and I'm intrigued. Is this some kind of wider convention, or just a one-off?