Single User runtime mode is the recommended choice when installing ArcIMS ArcMap Server. It is the only option that is allowed for installing ArcIMS, ArcReader or ArcEngine on the same machine where ArcGIS Server is installed.
When ArcEngine and ArcReader are installed in Single User runtime mode, they can be executed only by the user that installed those products.
Multiple User runtime mode should be used in the case where a single install of ArcGIS Engine and ArcReader is used by multiple system users. In such cases, ArcMap server installed on that machine can be configured to use the existing Multiple User runtime mode or can be set up to use a Single User runtime mode.
Multiple User mode requires Mainwin System Core (MSC) service installed as the superuser (root) user and this service must be running prior to the commencement of ArcIMS ArcMap server, ArcReader, or ArcEngine installation.