Depending on what contest it was, the reply you got
may have been accurate. Different contests have different exchanges. Some use serial numbers, some use names, some use locations or other pieces of information. There can also be a combination of pieces of information.
599 = signal report
PE = Prince Edward Island (PE or PEI)
13 = His serial number
During as contest the point is to log the information HE sends you. YOU would send HIM a 001 if he was YOUR first contact in the contest.
Next time, check out www.contesting.com
whatever contest begins. It will give you a list of the current contests (near the bottom of the page)
and there will be links to the rules for each listed contest.
IF for some reason contesting.com doesn't have the listing you are looking for (it IS very comprehensive)