What RelativeTo location should I use in FolderItem.SaveInfo?

If I’m storing a user chosen location, which could be anywhere (documents, network drive, external drive, application support), what should I choose as a RelativeTo base?

I’ve been using NIL so far but understand that may not be the best option in all cases. Can someone explain the benefits of the RelativeTo location?

Cross platform Mac/PC app.

Thanks!