This widget uses the Events API. You can read about BYU APIs here: https://calendar.byu.edu/byu-calendar-api-documentation
The specific API this module uses is: https://calendar.byu.edu/events-api
You can read more about the categories or see current values and information through: The All Categories API: https://calendar.byu.edu/all-categories-api
Or the Subscribe page which documents category and subscription options for Ical. https://calendar.byu.edu/subscribe
Category Ids used in iCal feeds and APIs are the same.