I have been thinking this over since the FAQs were released (to staff) and watching people react to it and I have come to the conclusion that this particular FAQ is not clearly written and applies to Advance tickets that are not routed '& Connections'. Imo, it is meant to give the following meaning.
Where the ticket is not routed '& Connections': The ticket must be used on the reserved services. If a portion of the journey has not been reserved, travel by any reasonable non-reservable service, that maintains a connection with the reserved service(s), can be used. If a reservable service is used during the unreserved portion of the journey, the ticket will not be valid.
Where the ticket is routed '& Connections': The portion of the journey reserved on the services of the TOC named on the ticket must be made on the reserved service(s), but connecting services may be any services that have reasonable connections, that can be found on a travel itinerary (for example from NRES), to those TOC services.
Example: A passenger is travelling from London to Paignton.
If they hold a ticket routed 'AP Slough' they must use the reserved service(s), any unreserved portions of the journey can use services that are non-reservable and still maintains a connection with the reserved service(s).
If they hold a ticket routed 'SWT & Connections' they must use the SWT service(s) that have been reserved (for the duration they are reserved), but may use any services that connect with the reserved SWT service(s) and can be shown on an itinerary.
I think this is about the best interpretation I can come up with that fits the T&Cs and the FAQs, though it may not be perfect.