SA: System and Services Acquisition – NIST SP 800-53 Revision 5.1 HIGH
NIST SP 800-53 Rev 5 HIGH SA-9(2): SA-9(2) External System Services | Identification of Functions, Ports, Protocols, and Services
Require providers of the following external system services to identify the functions, ports, protocols, and other services required for the use of such services: [Assignment: organization-defined external system services].
This control maps to 3 controls across 3 other frameworks. If you already hold one of them, the evidence you collected for it is the starting point here rather than new work.