The reservation procedure of CRIS is still unknown.
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Booking always starts from the middle of a coach and then it proceeds to the either direction. Once an application is received the software checks if it can put all the passengers together by fulfilling their desired berth options. (Providing sufficient number of berths is still available in the compartment=> coach)
If that is...
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not the case, the passengers would split up to the next available coach => berth choice. If berth choice still cannot be fulfilled, the system allots a random berth (Amongst the then available berths)
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Possibly in some cases, if criteria cannot be fulfilled in one single compartment and the reservation has not commenced in some other coaches, then perhaps system issues berths in the same coach itself. (It should not happen but possibly this is one of the reason)
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Also in LHB/Hybrid LHB, out of the 9 sets of SUB and SLB, 6 SLB are reserved for RAC (Usually there is 12 RAC per coach, since Duronto is for end-end and 12245 supports RAC whereas 12246 does not), chances of this being another cause to you being unsuccessful today is highly probable.
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You may also keep in mind that Duronto now supports Tatkal and perhaps some of the SLB and SUB could probably earmarked for Tatkal. Also count in HO quota.
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Now contradicting the first explanation provided above if I assume that for every Sleeper Coach, the remaining 3 SLB's (9 SLB in total - 6 for RAC) were all booked up, that is for 8 coaches (8*3=24) SLB's were all booked up (Or earmarked for Tatkal or HO), system tried to put you together (Or perhaps you had clicked the option of (Book my tickets if allotted in the same coach), and hence you were allotted LB/MB (Random combination) out of all the possible ones.
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If now out of 456 berths only 100 is available for booking, it would not mean that you may not get SLB/SUB anymore.
Reason:
Cancellation of confirmed berths.
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System usually do not swap berths between quotas. That is if one SLB was earmarked for Tatkal, it would supposedly be earmarked for the same and would not be swapped with any other kind of berths.a