In what type of situations ESB uses external web services?
I'm not sure, if I get your question right.
In what type of situations ESB uses external web services?
If an ESB process needs to access funcionality or pass data to a routine presented as an external web service...
Sonic ESB is communicating internally using Sonic JMS. This communication layer is kind of proprietary. So web services offers a way to talk to industry standard services.