My understanding is that, for regulated fares only, the train companies are compelled to offer at least the same route availability as was available in NFM 64 unless a Regulator has granted a dispensation for a particular fare, and that the train companies find it expedient to offer the same route availability for non-regulated fares.
From what I can establish, there was a certain wording in the National Routeing Guide,
"All fares comparisons must be made using the same ticket type. If
the Standard Single is the requested fare then compare Standard
Singles. If the Saver is the requested fare, then compare Savers,
and so on.
Sometimes (because of different local fares policies) a direct
comparison may not be possible. In that case Standard Single fares
should be compared. If a fare of the type requested has a lower fare
than all its routeing points, compare Standard Single fares.
Fares comparisons are made using National Fares Manual No 64.
Fares comparisons should match fares with similar fares routeings.
If that is impossible because the two sets of fares routeings are
not the same, compare fares for the cheapest fares routes."
However I am not sure when this was dropped.
It does not appear to have been here 14th April 2004:
http://web.archive.org/web/20040414012827/http://atoc.org/rsp/Routeing Guide/pdf_docs/sectionf.pdf
http://web.archive.org/web/20040417000439/http://atoc.org/rsp/Routeing Guide/pdf_docs/sectiona.pdf
but was quoted here in that same month
http://uk.railway.narkive.com/WNWUU...-ticket-destination-on-midland-mainline-etc.3
In September 2006 it seems ATOC submitted a revised RG Section A relating to the electronic RG implementation (in 2008.zip here:
https://www.whatdotheyknow.com/request/request_for_copies_of_every_pape#incoming-355100)
This however has been dropped for the 2004 version.
In addition, it appears the DFT never approved any of the other PDFs posted online, which apparently do not match the printed guide in many important respects. As I understand it:
The National Routeing Guide was a printed document
The Electronic National Routeing Guide was an electronic version of this, aimed at rail users, with some PDFs to explain how the system works.
The Electronic National Routeing Guide no longer exists, so the PDFs essentially no longer serve a purpose.
The online Routeing Guide at
http://www.atoc.org/about-atoc/rail-settlement-plan/routeing-guide is NOT the National Routeing Guide, as per the picture on page 1 here:
http://www.atoc.org/clientfiles/File/RSPDocuments/instructions.pdf which instructs you to use the National Routeing Guide
The National Routeing Guide is still a printed document, but one that is no longer available for sale, and the ENRG pdfs are used for this.