What does RAC stands for?

An RAC ticket gets you on the train (A WL ticket doesn’t) and in your
chosen class, but there’s a chance you could end up with just a seat.
(very unlikely if you book early)

A berth is split into 2 seats for 2 RAC ticket holders, if there’s
any last minute cancellations, or if any quota allocations remain
unsold, or if any confirmed ticket holders are given a free upgrade
(more later), an RAC ticket holder is given the empty berth, the other
RAC ticket holder can then convert the 2 seats into a berth.

RAC tickets are only issued for 2-tier air con (4 seats per
carriage), 3-tier air con (6 seats per carriage), non air con First
Class (4 seats per carriage), and non air con Sleeper Class (up to 12
seats per carriage).