Well, here is where the plugin is a bit messy. And that is mostly due to its history.
It started out life as a plugin which enabled Topic calendars. In this case, every reply on the Topic was an event (if it included a date/time). The Holiday calendar is a great example / use-case of this.
Subsequently, the ability to turn the OP of a Topic into an Event (displayed on a sitewide calendar) was bolted on; this has the somewhat odd name of discourse_post_event. This can be controlled per Category (in the Category settings) and restricted to groups via the site setting discourse_post_event_allowed_on_groups.
Additionally, you can elect to display a Calendar of these OP events (‘post events’) above a specified Category (will have just the Events from that Category). You can also enable (via a site setting) some fancy sorting of the Topic list of any Category by Event - this is turned on via a Category setting.
Judging by the recent chatter on meta.discourse.org about it, these per-Topic events now appear to be the dominant way to utilise the Plugin.
Personally, I find the settings that govern all this a bit of a mish-mash, with some confusing cross-over between site settings, category settings, and terminology.
I’d love to see this tidied up / rationalised. As it covers a key functionality for most communities, I’d love to see the plugin made freely accessible to a wider range of Discourse users (currently it is quite paywalled unless you are self-hosting).