Why use xsd.exe instead of SvcUtil.exe?

Svcutil.exe is a newer code generation utility that Microsoft has released as part of WCF in .NET 3.0.  Unfortunately, SvcUtil.exe does not support the more advanced XML schema features that are required by NIEM such as abstract types and substitution groups.

Source: IJIS Technical Advisory Committee NIEM FAQ Series "NIEM IEPD XML Code Generation in C# with .NET 3.5"; see article ID# 547