The reporter is any person or organization that tells a vendor or a coordinator about a possible vulnerability (3.5), once called the finder; nothing special is needed to be one, and reporters range from individuals and amateurs through professionals, end users and security research groups to other vendors and governments. ISO/IEC 30111 6.5.3.3 points vendors to this clause and to clause 6 for dealing with outside reporters: understanding what drives them and replying to them in good time.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.