The Show in SchemaAgent menu item causes the active schema and, optionally, linked schemas to be displayed in the Altova product SchemaAgent. (This product must be installed on the same machine as XMLSpy if you wish to use SchemaAgent functionality). The schema/s are opened in a new SchemaAgent Design in SchemaAgent.
Mousing over the Show in SchemaAgent menu item pops out a submenu with options about what schemas to show in SchemaAgent. These options are described in SchemaAgent in the DTD and XML Schema section of this user manual.