This year STEM Expo has drastically revised the categories to be more inclusive, and separate out multiple scientific fields into more specific sectional groupings.
We're keeping all the fun stuff, but adding a methodology to broaden our horizons as we continue to expand.
You can see what the new category descriptions are by following the link to the Category Descriptions.
The largest category in previous years has been the "Scientific Inquiry" category. This category is no longer available, but instead we've revised all the others to split that one out. We've created a document (here and below) that has some entries from a previous year's event split to the new categories as examples.
Finally, as the school systems progress to combining real-world and laboratory sciences, we are focussing the evaluations on some better defined core principles that are used throughout Science, Technology, Engineering, and Math. Read about those in the general rules and also at the Judging Info article.