The contest is the
ARRL Sweepstakes.
The exchange is a bit complicated and I think it's that way to counter the people who say a simpler exchange, like many contests have, is not a meaningful test of actually making a two-way contact. So, the first part of the exchange is a consecutive serial number (your first contact in the contest is your number 1 and you count up from there), followed by a single-letter "precedence" which indicates your entry category (“Q” for Single Op QRP; “A” for Single Op Low Power; “B” for Single Op High Power; “U” for Single Op Unlimited; “M” for Multi-Op; “S” for School Club), followed by your callsign, followed by the last 2 digits of the year of first license for either the operator or the station, followed by the ARRL/RAC Section (abbreviated).
For example, if I was operating single-up with 100 watts (low power) and I was responding to your CQ, I'd send "AC2OY 123 A W9BU 91 IN".
Each ARRL/RAC Section is a multiplier and if you are lucky enough to make contacts with every one (it's really tough to make contact with the Northern Territories in Canada since there are very few hams up there) that's called a "Clean Sweep".