It has changed.
They offer priority and pre-assigned seats. Both for an additional cost.
Then there is general boarding.
This is how it works....
Priority gets you an assigned seat, guaranteed carry-on space, and first boarding rights. I usually only see a handfull of people choosing this option.
Assigned seats...board after priority, they get a guaranteed seat for the first few minutes of boarding. They must board immediately after being called or risk losing their seat.
Then comes general boarding. No one has assigned seats after this. Our airport asks that families board first in an effort to keep them together. I have heard of airports that do not do this. Just make sure you are close to the gate when general boarding is called if you do not want a middle seat or need seats together.
At this time anyone with an assigned seat loses their right to have it, unless they are sitting in it. This is where I've seen problems. People assume since they bought their seat that it will be waiting for them, so they board towards the end of the boarding phase. They get to their seat and someone from general seating has now claimed it. The flight attendants explain they have lost their seat and people can get very upset.
I have a family of seven and we do not buy our seats anymore. We just board when families are called and never have a problem seating together.
The best advice is to sit close to the gate and attendant desk and listen to what ticket is boarding. Our airport does not allow anyone to be standing anywhere near the gate or they won't let anyone board. It just becomes chaotic when general boarding is called. You are at the best advantage if you get there early and snag a sit close to the gate.
Enjoy your trip!