Good open mat

What makes a good Open Mat? My friend's MMA school was going to
have an open mat but they weren't sure how to run it. They were going to
do it in the evenings for a few hours. Are open mats normally organized
in a particular manner? Or are they just a free fall for where people go up
to each other and just roll?

I thought the whole point of "open mat" was to have some space for people to work on whatever they want, with everyone deciding what to do by themselves. When I go to open mats, I usually grab someone who wants to drill and we work on some techniques and explore some moves, then spar a bunch. Some people just spar the whole time. I consider it to be a time for people to work on points that they don't get addressed in a normal class or to just put in more mat time. But it can mean anything to whoever shows up since it's their chance to pick what and how to train.

Thanks to all that replied. I'm kind of taking it day by day right now. I used to roll 3 days a week. The smallest things even help a little bit (aka taking the wallet out of the back pocket). Just found out that a family member just had the same thing happen to them. Their chiropractor told them it was just a muscle spasm. Seems a little ironic to me. This might sound REAL stupid but: can spinal muscle spasms be hereditary?