Copied from vSphere Security Guide
Deploying vCenter Single Sign-On in Basic mode means that a standalone version of vCenter Single Sign-On is installed on a system. Multiple vCenter Server, Inventory Service, and vSphere Web Client instances can point to this standalone version of vCenter Single Sign-On.
Basic mode means no High Availability or Multi-Site configuration.
In this installation admin@system-domain is granted Administrator’s privileges on vCenter.