This wait event is connected to heterogeneous services. Whenever we do something on a HS system from our Oracle session, we need to wait for remote system to do it’s work and produce a response. For Oracle it is an idle event as it’s not Oracle who does the work. However it should be remembered that performance issues on remote system will be passed on to our Oracle session and can be detected by large numbers on “HS message to agent”. It can be seen in v$session_event on 10g or higher Oracle versions.
The event is not documented well, and there is not many information about it on the net. I guess because it’s pretty self explanatory from the name. But people tend to forget that idle events don’t necessariliy mean they are not to blame for performance problems.