ATCO code starting with 9100 are formed by real TIPLOC codes.

[miklcct]

In the readme, it is explained that

There are a small number of NAPTAN nodes that do not have a TIPLOC, usually because they are a heritage railway location. For these we insert the value from the Naptan.ATCO field into the TIPLOC field.

All ATCO codes for the National Rail network are simply formed by concatenating 9100 in front of the TIPLOC code, and upon investigation of the data, I have found out that these codes starting with 9100 mostly corresponding to real National Rail stations which have been closed and removed from the network, including Canning Town, Farringdon (Moorgate branch), etc. If we remove the letters 9100 from the code, it becomes a real TIPLOC code which was historically used on the network.

For example, there is an entry of 9100PARISND in the file, but there is also a real TIPLOC code PARISND which is used for Eurostar services in the schedule data, and they represent the same location.