Difference between 'contained steps' and 'subpipeline'

I'm a little (ok a LOT) confused between the difference of 'contained steps' and 'subpipeline'

E.g in the spec in this part:
4.1

<p:pipeline
  name? = NCName
  type? = QName
  psvi-required? = boolean
  xpath-version? = string
  exclude-inline-prefixes? = prefix list>
    (p:input |
     p:output |
     p:option |
     p:log |
     p:serialization)*,
    (p:declare-step |
     p:import)*,
    subpipeline
</p:pipeline>


What's the difference between the sequence of p:declare-step and subpipeline ?
What really confuses me is this comment : from 2.1
------
Note
User-defined pipelines (identified with pfx:user-pipeline in the preceding syntax summary) are atomic. A pipeline declaration may contain a subpipeline, but the invocation of that pipeline does not.

------

So that last sentance ... is it saying the "subpipeline" in the above p:pipeline is not executed ?

If so what is done with it ?

Any references to examples would be greatly appreciated.

Thank you !



-David Lee

-----------------------------------------------------------
David A. Lee
dlee@calldei.com  
http://www.calldei.com