External entity interaction

An external entity represents a supporting domain or serves as a proxy for some actor within that domain. We can't send events to it since we don't presume that an external domain is modeled or designed in any particular way. We can only trigger synchronous or asynchronous operations. These operations, in turn, would map to whatever mechanisms are appropriate inside the external domain if code is generated from our models.

So we will see either single arrows representing asynchronous operations or solid arrows representing synchronous operations. A synchronous operation may return a value.