So...uhm...I guess this would be mine to answer. Sadly, it's all kind of confusing and sometimes vague.
An event, as defined by corporate, is any official/sponsored get together that is announced in the standard way. What is the standard way, you ask? That depends. For local meetings/practices/farbles, it can mean via email, webpage, FB, word of mouth, meeting announcement, semifore, smoke signals, kumi-daiko messaging. Whatever! For Kingdom events (those things we drive to on weekends and occassionaly for a whole week) must be announced in the Kingdom newsletter. The difference is that Kingdom Court business can happen at a Kingdom event, but not any old ordinary event. Royal events are the special Kingdom events described in Calontir law.
Demos are a wierd beastie. Since the local group announced it, it gets covered by insurance. Because it is wide open to the general public, it has a couple of special rules. They mostly come from the legal (read seneschal) side of things. Rather than listing the individual scenarios that have been made illegal over the years, the general rule is that the public cannot participate in combat related activities.
As it was related to me by the previous SocSen, someone threw a fit about allowing "mundanes" to fight in armour at renfest like demos, so that was made illegal. Someone found a loophole and started "whack-a-knight." Someone else threw a fit about that not being within the spirit of the rule, so that was made illegal. Someone decided that they would just have people sign a waiver, declare it a fighter practice, and then work with the public. Again, someone threw a fit and that was made illegal.
The general idea is that if someone walks up to the SCA at a demo and puts on armour, they might not really grasp the concept of what they are getting into. Someone who comes to a practice or an event has at least a basic idea that people are actually hitting each other and it isn't all just stage craft and fakery.
The insurance thing comes up a lot because it is an easy scapegoat that will assuage peoples objections. The SCA insurance does not cover injury. If anybody gets injured during an SCA activity, they must rely on their own insurance. The SCA insurance is mostly for property damage caused by the activities of the event, although we all know that it has other uses for protecting the corporation.
What if they sue, you ask? Well, (speaking as a teacher and not a lawyer here) I'm pretty sure that the courts have set clear guidelines on that one. The courts have regularly held that certain sporting activities have inherent dangers and that participating in the activity restricts (or completely negates) your ability to sue over injury. We use the waivers as proof that the parties involved knew that it was possibly dangerous.
So, here's the short version.
A demo is covered by insurance, but the rules say we can't put people in armour or let them try it out until they come to a practice or event.
Murdoch, the guy who is almost done and won't have to answer these type of questions any more.