They evaluate all the information they have obtained about you from the CFAT scores, the fitness test, the interview, your personal references, and anything else that they know about you positive or negative.
They then assign you a merit score based on their criteria and how you fit into it. Assume that the merit listing scores range from 1-10, 10 being the best. They hire people from a Canada wide list of potential candidates, starting with all those who earned a 10 and then going backwards from there.
If you have a very low merit score in a popular trade, you might have to wait for a really long time.
You don‘t get to know what your merit score is (as far as I know) but you should have a pretty good idea of what it is in terms of poor/average-above average/ and superior.
You can assume that you‘re going to get in for sure if you know that you‘re in the superior range.
Probably most people get in from the above average range, and some from the average, as long as there is big intakes for that trade (ie combat arms, and the Navy).
If your score is so poor that it is certain that you will never receive an offer, they tell you so even before the file goes to Borden.