How does rush work? Different forms of rush?

[Flying Tiger]

Hi everyone. I am confused on rush.
At my school, rush is held at the beginning of each quarter. Each house hosts their own events and you can attend as many events from as many different houses as you want. There is little to no coordination between the houses and each house basically conducts rush independently from other houses. All events are open invitations so you don't need to be invited back to attend the second events. You also don't need to sign up with any Greek councelor.

Then I hear others speak of a more structured form of rush where the houses all work together. On the first round, you attend all the houses and stay for a few minutes and then move on to the next. Before the second round, you have to eliminate a few houses and the houses also eliminate rushees so you have to be invited back for the second round where you stay a bit longer at each house. The process of continues until you pick a few top choices and the houses also chose their top rushees and you basically hope that you get offerred a bid from a house of your choice. This seems like a very structured form of rush with lots of coordinating between the houses and the university. Also some people say that this happens in the fall and if they don't get enough people they do some do some type of informal rush in the spring.

Can someone please explain to me these two very different forms of rush as I feel I have missed something. I know my school does not have a day where you attend multiple houses and you need to be invited to return the second night. Also can someone clarify what it means between formal and informal rush? Is it what I described and why is one in the fall and informal is in the spring but only if they don't reach the maximum memebers. Who keeps track of the number of actives in each house?