Your year you or your station was first licensed.
e.g. I got my first license in 1978, so for my check I send 78. Whatever you send, make sure you send it consistently.
Full SS rules at http://www.arrl.org/files/file/Contest%20Rules%20PDFs/2012/2012-SS-Rules.pdf
4. Contest Exchange: The required exchange consists of:
4.1. A consecutive serial number;
4.2.1. “Q” for Single Op QRP (5 Watts output or less);
4.2.2. “A” for Single Op Low Power (up to 150 W output);
4.2.3. “B” for Single Op High Power (greater than 150 W output);
4.2.4. “U” for Single Op Unlimited Single-Op Unlimited High Power and SingleOp Unlimited Low Power both send "U")
4.2.5. “M” for Multi-Op (Multiop High Power and Multiop Low Power both
4.2.6. “S” for School Club;
4.3 Your Callsign;
4.4.1. The last 2 digits of the year of first license for either the operator or the
4.4.2. An entry must send the same Check throughout the entire contest.
4.5. ARRL/RAC Section (click here for the official list)
4.6 Exchange Example: NU1AW would respond to W1AW’s call by sending: W1AW 123 B
NU1AW 71 CT, which indicates QSO number 123, B for Single Op High Power, NU1AW, first
licensed in 1971, and in the Connecticut section.)