[platform-ui-dev] Non-extendable interface?

[Jared Burns <jared-eclipse@xxxxxxxxx>]

The JavaDoc on the interface 
org.eclipse.ui.externaltools.model.IExternalToolRunner says:
"This interface is not intended to be extended by clients."

We have plenty of interfaces that aren't to be "implemented" by clients but 
I've never seen a comment about clients "extending" one. Can you explain the 
purpose of this restriction?

Thanks,
- Jared