National Routeing Guide
""Ticketing arrangements on the UK's railways are notoriously arcane, but you can use the rules to get cheaper fares. "
The routeing guide makes possible some of the recently publicised ticketing anomalies in the UK rail network such as saving money by purchasing tickets for long journeys as several discrete journeys instead, for example. As such it is a powerful tool in the arsenal of the knowing consumer, given the current complexity of ticket choice on the UK rail network. It is also important for travellers who contemplate a different route when faced with disruption; under these circumstances staff may stamp or sign a ticket as valid via a different route. When it was introduced, its primary aim was to "provide passengers with certainty about what travel their ticket buys them" ", after many years of ambiguity over 'reasonable' journeys, therefore passengers are now quite within their rights to use it as a point of reference, since it has been written and approved by the transport companies. The primary concern of those travelling is staying on a route acceptable for the ticket they have purchased. The customer is always on-route if they can answer yes to at least one of the below, according to section A of the guide:

The full routeing guide is consulted only upon answering 'No' to both questions. This can occur when the customer wishes or needs to change trains several times, either with a view to travelling on a particular line (for example to connect with a faster train), or because of a desire to break a journey at a given station not directly on the 'expected' route. Passing through the same station twice is almost always forbidden, except where a rule or 'easement' allows it. These easements are exceptions to the acceptable routes which are otherwise explicitly defined, and are listed in Section E of the Guide for easy reference. They are simple to understand one-line rules, and exist to ensure that in most circumstances the simplest journey is acceptable. For example, the "no doubling back" rule normally requires travellers changing from one line to another to change at the junction station. However, such stations are often small and poorly served, so local easements often exist to allow travel to the nearest major station. In many cases this enables the traveller to remain on 'fast' services. "Easement 700221: Customers travelling from, to or via Truro to St Ives ... may double back between St Erth and Penzance. This easement applies in both directions. "
Others are matters of convenience:
"Easement 020003: Passengers for Aviemore and Carrbridge are permitted to alight from the sleeper at Inverness and double back to their destination in the morning. "
Some easements are negative, forbidding a route that might otherwise be acceptable. Some may be both positive and negative: 
"Easement 000026: Customers travelling from Micklefield, Garforth, East Garforth, and Cross Gates to or from London, route YORK, may travel either route YORK or route LEEDS. Route ANY PERMITTED only allows travel via LEEDS. "
The relevant section of the Guide, which is currently available at the Rail Delivery Group website, details more than five hundred different easements. Train Operating Companies may make special arrangements for disabled passengers, who have further exemptions on an individual case basis. This allows for different routes in certain situations, such as where normal practice is to walk between two nearby stations on different lines to catch a connecting train, which wheelchair users might find difficult. There are however no clear stated rules for defining what is "reasonable" for disabled people—this is presumably a matter of discretion. Some journeys must involve walks of up to 10 minutes in some cases between stations to make a connection (e.g. Farnborough (Main) to Farnborough North, Ash Vale to North Camp) Three types of routes are acceptable: "direct trains", "shortest route", or "mapped routes". The first two are simple and outlined above. Almost the whole of the routeing guide is taken up with specifying the third for the entire country. Principle

Journeys
The rules can be summarised thus:

The guide allows many journeys which one might not expect. Travelling from Cardiff to Cambridge via Swansea, Shrewsbury, and Birmingham is acceptable, for instance, rather than simply via London. Generally there are a large number of permitted routes which are rarely used because they are inconvenient, but which are nevertheless legitimate. Some travellers have reported being charged extra for 'special' routes, however.
Issues of value to the traveller
As stated by the Rail Regulator, "[it] sets out passengers' rights to use the network flexibly". It is also always on hand to staff at manned stations. Since most rail travellers make 'simple' journeys, many will never encounter it. However, when making more complex journeys, it is important to stay "on route". The guide defines what this means. Individual tickets may restrict holders to a subset of the acceptable routes, usually by requiring them to travel through a particular station.