Can you reconcile those two seemingly conflicting statements?
They both say the same thing.
An FTD opens when a sale has not settled within the grace period (3 days??) it stays open until the sale settles.
An NSS short can never settle, so the FTD would have to stay open forever. If an FTD has closed then the sale has settled. Meaning it cannot be a NSS. Simple.
Two choices, if a sale has settled the FTD goes away and the sale cannot have been a NSS. If it is an NSS it can't ever settle and the FTD count includes the NSS count.
What do we know then? We know that the NSS (if it exists) cannot be any higher than the FTD count.