It is licensed with vCenter. Technically it requires you to have a valid vCenter license key. Depending on the vCenter License Edition, vCO will run in Player mode allowing you to run but not edit workflows, or in Server mode, allowing you to run and edit workflows. vCO runs in Player mode with the vCenter Essential and vCenter Foundation licenses, and runs in Server mode with the Standard and higher licenses.
Player allows you to run the workflows shipped in the library and run third party workflows. Server allows you to access the editing functionality so that you can create your own workflows from scratch or  duplicate and modify existing workflows.