ArcMap Server 4.0 and 4.0.1
ESRI supports whatever is exposed in ArcGIS and ArcMap Server, except all the CoClasses, Interfaces, and Enumerations that relate to the application framework and application extensions. For a list of data types and members contained within esriCore.olb that are not available to developers in the ArcIMS ArcMap Server extension, click the link to the ArcObjects Online technical document in the Related Information section below.
ArcMap Server 9.x
ESRI supports all customizations that use the following ArcEngine libriaries: Carto, DataSourceFile, DataSourceGDB, DataSourceOleGDB, DataSourceRaster, Display, GeoDatabase, GeoDatabaseDistributed, Geometry, GISClient, Location, NetworkAnalysis, Output, Server, and System. ArcMap document customization that is reliant on anything outside of these libraries is not supported by ArcMap Server.
Any customization must be present in a new COM object. ArcMap Server does not support VBA code embedded within the map document.
Custom behavior or custom implementation of custom features and/or custom layers is not supported.