Everyone is a gangster (Rajdhani, Shatabdi, Tejas) until the real Gangsta (Vande Bharat) walks in 😂 - Mushfique Khalid

Hi all! I want some clarifications regarding Wait list quotas since I don't have any wait listed ticket booking experience. It would be really helpful to me if you answer in a clear and simple manner for the following queries. Thanks in advance. My queries are :
1)What is a Waitlist?
2)What does PQWL mean?
3)What does GNWL 100/WL 50 mean ?
4)What...
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does PQWL 100/WL 50 mean ?
5)What does GNWL/Available mean ?
6)What does PQWL/Available mean ?
7)What is RAC ?
8)What does RAC 25/RAC 10 mean ?
9)What does GNWL 50/ RAC 25 mean ?
10)What does REGRET mean ?
11)What does REGRET/WL 1 mean ?
12)What does RLWL 50 / WL 25 mean?