Yeah, I've been wondering about that myself. I wonder if it's something like, "if there's an existing story with the same name, you can check this box and any contributions will be synced there?" But that's just a rough guess.
I tried using the contact form to ask about it but got radio silence so I'm gonna try and ask about it using the email they mentioned in the recent TOS update (
help@mycast.io) and see if that gets any results. I know they've been doing some behind the scenes updates to prepare from the upcoming site revamp just by accidentally catching them on-site...