The ArcGIS Server Administrator Directory is a web application primarily designed to help you write administrative scripts for ArcGIS Server. You can browse the Administrator Directory to learn how to construct administrative web service requests through REST. You can then formulate these requests and send them over HTTPS using a scripting language of your choice.
The Administrator Directory is typically available at https://gisserver.domain.com:6443/arcgis/admin.
Learn more about scripting ArcGIS Server administration
You can also use the Administrator Directory to perform certain administrative functions not available in ArcGIS Server Manager. For example, to create a new geometry service or search service, you must use the Administrator Directory.