what is the difference between a trigger and user defined event?

In postgres there is a clear distintion between trigger and event

trigger. Triggers are activated when DML statements are executed whereas event triggers are activated when DDL statements are executed. 

But in soliddb (from which our post in JIRA page is inspired), it seems like user defined events can be activated for DML statements. What is it that can't be done with triggers and can be done with user defined event?

Why is the restriction that events can only be posted and received by stored procedures only?