confused on tiers

Tiers basically means how competitive they are at recruitment. More people want to be in the 'top tier' so they're more competitive.

Unfortunately, people like to equate it with looks/partying, but that's not really what it should mean, persay

Yeah, "top" and "upper mid" are all the same tbh. There is no difference, really, between them. Top tier sororitys party with middle tier frats, and middle tier sororities party with top tier frats all the time.

Don't really listen to this site.

You just listed sororities that are all in the same tier with the exception of KD and Chi Omega.

Pi Phi, DG, Alpha Phi, Theta are in the same tier. People will rank them differently based on personal preference but it doesn't really matter since they are in the same tier. You are correct at the end of the day they more or less have the capability of having social events with any fraternity they want. Typically this means they will have TG's, block and other exclusive events with mostly top tier fraternities.

KD/XO are generally considered upper-mid tier which means they will have TG's, football block and other events with a mix of middle and top tier fraternities.

The core top tier only consists of four sororities now. Just for variety's sake eventually top tier fraternities do change it up a little bit and will have some events with upper-mid to mid-tier sororities as well.