DefCore defines the capabilities, code & tests productos labelled as OpenStack must pass, thus creating a standard and the minimum requirements these solutions must meet.
It is a way of increasing user's trust guaranteeing that those projects which aren't mature enough to become part of OpenStack's ecosystem are filtered if they don't meet the defined standards.
And who is in charge of this task? A 17 members comitee formed in 2013 and supported by the community establishes, updates and mantains the interoperablility guidelines.
They state the components and capabilities an OpenStack product must have to ensure interoperability and stability to the users. The components must integrate OpenStack code and capabilities are checked by API tests.
For further information read the source article at OpenStack SuperUser blog