Upvote:5
Indian Railways has approximately 1 bazillion different quotas (Tatkal, Foreign Tourist, Handicapped, Ladies, ...), but as far as I'm aware, online bookings are not one of them: both ticket counter and online bookings use the same quotas. If you're getting different answers online and in person, you're presumably looking at different quotas/tickets/trains/classes?
For more about the algorithm, this Quora answer goes into far more detail than I can attempt to summarize.
Upvote:5
Indian railways is having many quotas. There are 18 in total as listed here
Out of these only 4 quotas can be booked online (premium tatkal is not a seperate quota. So it is not listed here). They are
General Quota
Ladies Quota
Tatkal Quota
Physically Handicapped Quota
So if you are booking on a different quota, for example Foreign Tourist Quota then you can buy the tickets in counter while your online booking may say no tickets are available
.
There is no seperate quotas for online booking and counter booking. It is just that you cannot book some quotas online.