An agent desktop application is a required component of an IPCC deployment. The agent uses this application to perform agent state control (login, logout, ready, not ready, and wrap-up) and call control (answer, release, hold, retrieve, make call, transfer, and conference). In addition to these required features, the application can provide enhanced features that are useful in a contact center environment.
http://www.cisco.com/en/US/products/sw/custcosw/ps1844/products_implementation_design_guide_chapter09186a008057a616.html
