Closed Loop is a relatively straightforward concept that is document in the Service Strategy volume of the ITIL version 3.
Closed Loop describes the fact that to create value from a service providers perspective requires increase service potential. The service potential is the ability of the service provider to utilize their capabilities and resources in a way that would allow the customer to achieve particular business outcomes.
The corresponding value point from the customer perspective is called the Performance potential. It is this potential that allows the business to attract and retain commercial customers - so driving up the demand for services from the service provider and reducing their idle capacity.