Solution
SystemLink uses port 12100 for communication with the User Service.
The SystemLink User Service is responsible for managing workspaces, users and permissions. It listens on port 12100 internally (it is not exposed publicly). When other SystemLink services need to communicate with the User Service for authentication/user information, they will do so on port 12100.
If external API requests are made to the User Service, the NI Web Server listens to requests on the normally configured port, and forwards these requests internally on port 12100.