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.