I do apologise if my rather rushed post this morning misled. I was passing on
RTT» 's "official" explanations, assuming their wording obviously havd nothing to do with "The Railway".
There is accessible support information for the data feeds in the
Open Rail Data Wiki, but it isn't up to date.
The Schedule feed contains the
WTT▸ entries, which are there from one timetable change date to the next. There is an entry per service, indexed by UID number. Services added during the timetable's currency have STP entries. These are "targeted" to be in place twelve weeks in advance, though obviously some happen much later than that.
Changes are made by added entries with the same UID, and these apply on stated days only. Adding a CAN entry means a service does not run on that day. Adding a VAR entry means the service runs, but with differences.
Very Short Term Planned entries come in a different data feed (called VSTP). They use the same format as in the Schedule feed; they are interpreted as overriding those. No more detail is offered by the Wiki. I think they were originally expected to all be new entries, i.e. equivalent to STP but too late for one. Now it seems they have discovered that the other entry types are also possible and are using them. The "V" in these means "Very short term", i.e. in the last 48 hours.
VST new services added in the VSTP
Anything in the Schedule - WTT, WTT+VAR, STP, and I think STP+VAR and even VST, can be altered by VVR or cancelled by VCN.
My take on this is that the 48 hour cut-off was the last time a train schedule could be sensibly altered, as at that point the schedule was turned into train plans (allocate stock and crew, plan station movements, etc..). Any later change or cancellation was dealt with operationally, as a deviation from the current timetable. But a new train was still worth adding, so that everyone at least had a UID to work to. Since it wasn't in the train plans yet that wasn't a constraint; and it would have to be improvised into existence in any case.
If that was how it worked in the olden days - say five years ago - now the
TOCs▸ have a need to change the schedule very late so as to make their cancellations vanish. Thus they are now using these new entry types, for which the new codes are appearing in RTT (where they were invented, I think). This evolution is not reflected in the documentation available (of course much of that was not intended as public).
Another evolution: The VSTP feed is updated in real time; the Schedule feed is only updated overnight. Logically, there should be nothing in the Schedule feed that is new and applies within the next two days. However, the Wiki implies that may happen, so I guess realtimeness has been spreading throughout the system.
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