There was a thread where we talked about W/L/Available/RAC etc and how it works. Here is an answer provided by a person who actually had a design on this:
Quote:
> About Regret after waitlist has started.
Waitlists are maintained at the Train-Date-Route-Class (TDRC) level as also
certain...
more...

quota-Roadside station and pooled quota levels. What I have given
below is for the TDRC waitlists. Numbers in the examples are from pre-1995
version of the software when maintained by CMC and for illustration only.
At the time of train introduction / firing, a max value of waitlist is
assigned. It is mostly the judgement of the Database Administration IRS
Officer and the experience of the booking operators and quota entry folks
which is used to decide what the size of the waitlist is going to be.
So consider a case where there are 12 Sleeper coaches in a TDRC. The limit
of the waitlist is fixed typically at about 300 or so.
Till such time the RAC is not started, you get a confirmed ticket with coach
and berth number.
Consider the following cases:
1) RAC starts and no cancellations - RAC number is given.
2) RAC starts and moves to number 5 and 10 confirmed tickets are cancelled -
RAC/CNF status given, coach/berth number NOT given.
3) WL starts, max RAC count in the TDRC is 80 and there are 70 cancellations
- WL/RAC status given, coach/berth number NOT given.
4) WL Starts, WL current number is 10 and max RAC count in TDRC is 80 and
there are 100 cancellations - WL/CNF status is given, coach/berth NOT given.
5) WL Starts, WL limit is 300 and WL running number is 299 but current WL
number is 200 and you get a 3 passenger transaction - First two get
WL299/WL200 and WL300/201 and the third is declined with a RGRT.
Now let us say after #5 an HO (High Official NOT Head Office!!) quota
passenger comes. Allotment of HO quota happens only when there is a PNR
number. Typically if WL has not reached WL, then the person is put on WL
and he is allocated to HO quota from WL. But what if WL limit is reached?
There is a provision to extend the waitlist, make a booking on waitlist and
then allocate the person on HO quota. Exten by how much? Typically by 5 or
so. They extend and immediately enter so that it prevents other operators
who may get a request at precisely that moment for the same TDRC and may
find available to put on WL and block that extended WL capacity.
Across the system, there is a limit of the maximum value a waitlist can
take. In my time (pre-1995) it used to be 600 so we fired trains with only
300 as the WL count at the time of firing. If we were to fire the train
with 550 or so, we would have limited capacity to extend for HO.
The same extension of waitlist function is used when there are an unusually
high number of cancellations and it results in current status coming to
RAC. At that time the waitlist is extended in much larger counts than just
for HO quota.
To answer the question - (finally! laughing!) - yes there is scope to
restart the booking but only explcitly on judgement.
Cheers!
Unquote
The person is a member of this site at my invitation, but wishes to remain anonymous again.. Please see my blog posting regarding PNR working.
Good wishes.