Answer
The terms public and private are used to distinguish the ArcIMS Virtual Servers. The Image, Feature, and Metadata* Virtual Servers are considered public, while the Geocode, Query, and Extract Virtual Servers are private. The extensions for ArcIMS, Route Server and ArcMap Server*, add two additional public Virtual Servers.
Public Virtual Servers can be directly specified by the Web developer. These are the only choices presented when using either ArcIMS Administrator, Service Administrator,* or Manager for site management.
Private Virtual Servers are used automatically when the ArcIMS Service requires the explicit functionality of the private Virtual Server. For example, if an Image Service contains a geocodable street network and the Geocode tool is included on the Web site, the Geocode Virtual Server is used with the Image Virtual Server.
*ArcIMS version 4.0 and later.