with-input name="source"

Greetings XProc friends,

In Erik's book (p 126) it says that if we leave p:with-input with no @port specified (XProc 3.0), it will default to the primary input port of the step, so I can have

<p:identity>
  <p:with-input>...</>
</>

And get what I expect. (Awesome.)

But it also says that sometimes it's different from this.

Could I have an example of a step where it's different?

Is there a way I can tell in the spec?

Many thanks,
Wendell