Open Stack

I'm looking at the documentation at 

  http://docs.openstack.org/icehouse/config-reference/content/section_compute-scheduler.html#computecapabilitiesfilter

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ComputeCapabilitiesFilter

Matches properties defined in an instance type's extra specs against compute capabilities.

If an extra specs key contains a colon ":", anything before the colon is treated as a namespace, and anything after the colon is treated as the key to be matched. If a namespace is present and is not 'capabilities', it is ignored by this filter.
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I've read that section several times now, but I don't understand.
What does it actually mean??

What is "an instance type's extra specs"? And where do I set it?
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