The term Change of Status (COS) is the name given to the process through which a person can change from one temporary status (nonimmigrant) to another temporary status without having to leave the United States. Typical examples of this process are changing from a student visa status to H-1B. Usually, a successful COS is evidenced by the USCIS issuing you a new I-94.
Changing status from a temporary visa to a green card within the United States is also a change of status, but it is technically referred to as Adjustment of Status (AOS).
The US State Department has made the following announcement today regarding the phased resumption of visa services.
Q: When using the new visa appointment site, what should my client list in the mandatory field that asks for the "Request Number"?
A: When a visa applicant sets up a user account in the appointment system, an 8-digit identifying number is assigned to the applicant. When logged into the appointment system, this "Request Number" appears in white against a red background in the upper right hand corner of the screen in parentheses after the applicant's e-mail address: