Change requests are common outputs from multiple PMBOK processes (often times, those which are part of the Monitoring & Controlling process group) but all become inputs of the Manage Integrated Change Control process which results in them either being rejected or turning into Accepted Change Requests.
In general, most of the M&C processes will take one or more "plan" inputs and compare those with one or more "actual" inputs. If there is a variance, one possible outcome is the issuance of a change request.
Kiron Saving Changes...
Sergio Luis ConteHelping to create solutions for everyone| Worldwide based OrganizationsBuenos Aires, Argentina
Are you asking from the point of view of what you have to answer in a question inside the PMP certification or in the real life? If the first then take a look to PMBOK. If the second then it is about your project change request process. Saving Changes...