What do numbers signify in airport codes? As an Air Cadet, we go gliding very frequently. However, I've noticed something about the airport that we go to. The airport that we go to, Markham Airport, has a different airport code. CNU8. 

Meanwhile, other airports, such as Toronto-Buttonville, Toronto-City-Centre, and Toronto-Pearson all have the following codes respectively: CYKZ, CYTZ, and CYYZ.

Do the numbers in airport codes signify something? If they don't, why is there a mix of both letters and numbers?
The code for Markham Airport, CNU8, is a Transport Canada identifier. 

The codes for the other airports are ICAO airport codes. ICAO airport codes always consist of 4 letters. Markham Airport does not have an ICAO airport code.