I don't know what to write.

When I see the translation of transient, I see it means "ephemeral" or "momentary". But every events are ephemeral! That is why I wanted to call it "Hidden". But then, we can think "why do we send an event which should not be seen?".

To me this tag means: do not show this event to the users, and do not show events related to it (when it is coupled with begin/end).