For a traveller arriving at, for example, a London terminal station and intending to go to Penge, it makes little difference which route is chosen.
Therefore, a ticket issued specifically to one of the Penge stations would be unduly restrictive—it would remove the opportunity to travel by a choice of equally convenient routes.
[1] The concept is explained in the National Fares Manuals (NFMs) issued approximately three times per year by the British Railways Board (and, since privatisation, by the Association of Train Operating Companies) to stations, Rail Appointed Travel Agents and other ticket issuing authorities: "Fares for certain ... cities and towns are shown to and from a notional common station[.]
[3][4] After privatisation, the designation "BR" was no longer appropriate, although having been in use for more than 10 years it had become a convenient shorthand device for referring to the station groups concept in general (for example, National Fares Manuals continued to use the term "BR Stations").
The solution, introduced gradually from November 1997 and uploaded to all ticket issuing systems by January 1998, was for BR to be changed to STNS or STATIONS as appropriate: There were some deviations from this standard: Additionally, the London group is treated as a special case.
[28] Rail tickets in Germany can be issued to station groups in many cities, referred to as Zielbahnhöfe mit tariflicher Gleichstellung (in effect, "destination stations with equal fares"), so long as the total travel distance for the ticket is greater than the threshold set for each city.