I am going to decline to answer this (sorry!) until an actual example is given, with named stations and exact tickets held (including ticket type, origin, destination).
I could go on a huge "if this, if that" response, but it would be a waste of my time, and a waste of time for people reading it. Sorry.
Well, here is an example.
Passenger is travelling Leeds to Manchester Airport. They have a West Yorkshire day Rover covering as far as Marsden, and a Greater Manchester Day Rover covering from Greenfield into Manchester.
The bit that is missing is Marsden to Greenfield.
If they were travelling on the hourly stopping service then the UFN would be from Marsden to Greenfield.
If they were travelling on the TPE service which runs non stop from Huddersfield to Manchester then the UFN would be from Huddersfield to Manchester as the West Yorkshire Day Rover ceases to become valid at Huddersfield as that is the last station the train stops at in West Yorkshire. Manchester Piccadilly is the first station the train stops at in the GMPTE area, and so the GMPTE ticket does not become valid until that point.
As you say though their are all sorts of different outcomes depending on a) what actual ticket types are held and b) the stopping pattern of the train.