How to understand a event stream equals to a aggregate function?

Recently I was using EventStore. The concept of event stream in my mind is that a event stream contains all domain events happened in a aggregate. so a stream per aggregate.

However, EventStore told me that stream equates to an aggregate function(here is document), that confues me. Does an event type equals to a stream? An event type also equals to an aggregate function?

You can use a stream per aggregate, this is in fact a common way of doing things.

The word function can be a bit confusing, but you can see the aggregate as a function from state and event to a new state, look here: