Back in the late 90s/early 2000s, there was an ACHA M2 conference called the University Hockey League.
https://en.wikipedia.org/wiki/University_Hockey_League
IUP's first team and Pitt were mainstays in the UHL, the rest of the membership fluctuated. Both Pitt and IUP were in the WPCHA, which preceded CHE. I believe that just about all of the teams in the area were in the WPCHA/CHE at some point in their history. Around 2006 Pitt and IUP moved up to M1 and the UHL folded. By then, just about everyone in Pittsburgh was either M1 or M3, so it kind of made sense. RMU, W&J, and SRU had jumped to M1 from M3 a few years before that, and WVU, Duquesne, and Mercyhurst had been M1 for years.
I haven't followed the ACHA for a while. However, back in the 2000s/2010s, the difference between M1 and M3 was more the level of commitment by the teams to things like game times, amount of practice, length of the season, etc. Maybe you could it professionalism and just generally how the program was run. The M1 teams were/are generally more stable, and had a lot of non-students running the show, while the M3 teams were often at least partially student run. The talent was definitely better on average in PGH M1 teams vs M3. However, there were plenty of good players on M3 teams and Cal circa 2006-09 in particular sticks out in my mind as a team that was better than most of the CHMA.