Say for a train from Howrah to Mumbai via Pune,
if Howrah-Mumbai and Howrah-Pune ARP Availability is 100 and Pune-Mumbai is 50, as soon as one books Howrah-Pune (Since the same berth availability has already been pre-defined till Mumbai), Pune-Mumbai availability status would rise by 1.
This phenomenon happens in only few trains and applies for both General and Tatkal bookings. One would need to carefully study the availability status for the ARP dates and its surrounding days to understand this.
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How to understand?
Pune - Mumabi ARP Availability for all the days is "2". However on some specific dates it is either 3 or 4 or 8 and so on.
Reason => Berths booked till Pune has been forwarded from Pune onwards.
Other side of the Coin=> We all know the concept of Confirmation of berths from charting at originating station or previous remote station.
In this case, the same concept as described above applies. Only exception is that berths are forwarded during charting as the same berth is not defined beyond a specific point during booking.
Hence, if someone books from Pune - Mumbai (Wl'ed ticket), he may get a Confirmed berth during chart preparation at Howrah (Owing to vacant berth arising from Pune onwards => Confirmed berth booked for Howrah - Pune forwarded from Pune onwards)