The first selection process is at the end of Primary, where based upon your grades and what is available, you get orders to your next phase of training. The choices are Jets, Maritime(Props and the E6), and finally Helos(come on down to South Park..).
In the case of Maritime, you will automatically know what you will be flying, in my case the P3, but for Helo and Jet training you still have to go through another selection process for the specific aircraft you want, once again this is based on your grades and what is available.
Here are three links to previous discussions on the forum about the selection process and NSS, hope that helps:
Primary to Jets
Jet NSS
What is this friggin' NSS?
Edited by - John Wickham on 8 August 2000