Not an expert on ticket buying, but from what I understand Gala and Special Presentations are the two categories of movies with Premium screenings. This is usually only the first 1-2 screenings, and is where the talent (directors, actors etc) attend. After these are done, the movie will have some Regular screenings, which will not include any talent and will be, well, a regular screening. With the regular flex pass you cannot choose films that have a Premium on it, only the regular screenings. So yes you are somewhat correct about that.

The main difference with Premium Screenings is that they are attended by talent, and are often the World/North American/Canadian Premiere. They definitely have a more lively atmosphere, especially the Galas which are the ones with Red Carpets and heavy Press attention. Attendees are more excited, some dress up real fancy (while others come in casual outfits) etc. Just note that if if buy a ticket you likely won’t be able to also watch the red carpet, but you will be able to see them on stage. For me, Regular screenings are maybe as exciting as going to see a big blockbuster movie in your local theatre year-round, just maybe in a fancier venue. They’re still great, but personally I’d skip it if it was something like A Star Is Born that was widespread a few months after TIFF, and choose something that would be harder to find.