The individual pieces of data flowing through Vector are known as "events". Events are arbitrarily wide, and deep, structured pieces of data. They have no requirements or limitations. Ideally, events contain enough rich information to derive any type of monitoring data from it.
Vector defines subtypes for events. This is necessary to establish domain specific requriements enabling interoperability with existing monitoring and observability systems.
Why Not Just Events?
We, very much, like the idea of an event only world, one where every service is perfectly instrumented with events that contain rich data and context. Unfortunately, that is not the case; exisiting services emit metrics, traces, and logs of varying quality. By designing Vector to meet services where they are (current state), we serve as a bridge to newer standards. This is why we place "events" at the top of our data model, where logs and metrics are derived.