Thank you for your helpful replies. At least we can be definitive on the fact that it is not definitive.

So, perhaps a safely worded rule is that "A stopover can definitely occur at any city that is part of a published route (online) from the origin to the destination in either direction. However, in certain cases, other cities can also be used as stopovers. These 'other cities' must be confirmed as valid by a Delta representative at the time of booking".