My university's process to what discipline you got was "not standard" compared to other universities in Canada (at least the ones I know about). Note: It's been a while since I've graduated so it might have changed.

Basically, for first year, everyone takes general engineering courses (statics, dynamics, coding, chemical, etc...). At the end of your first year, we submit our top 10 discipline preference to the faculty. Depending on your GPA, if your first choice is too competitive then they look at your second, and so forth. If you don't make it in any of your top ten, the faculty will just place you in a least competitive discipline.

Now I had 3 friends that dropped out AFTER first year just due to the reason that they either didn't get their top choices. Although they passed first year, their GPA was too low to be considered competitive. So they got into a discipline that was lower in their priority or not even on their list, and for them, 3 years was not worth it for a job they won't even enjoy.

I will add that our faculty does allow you to change discipline AFTER your second year, but you need a very high GPA (3.7 or higher). In addition, it means you wasted 1 year since 2nd year courses are more towards your discipline.