SCIM is a REST API-based protocol. Requests for SCIM are performed via HTTP requests (GET, POST, PATCH..) and need an HTTP URL. Even if the application is hosted "on-prem", it needs to have an HTTP server running to handle the HTTP request/response processing. The URL doesn't need to be externally resolvable, but does need to be accessible to the provisioning agent and resolvable via the internal DNS available to the server the agent is running on.