What are Domain Model Libraries
One of the new features in VS2010 is the notion of Domain Model Libraries. This is the equivalent for dslDefinition files, to the class libraries for assemblies. You can have, in a Domain Model Library types, (external types, enumerations), DomainClasses, and even shapes, that you will use, reference or inherit from, in your Dsls.
The very interest of the Domain Model Libraries is when you have several DSLs that share common types, that can, therefore be factored out.
How to use domain model libraries?
Eyal has already blogged about how to create a DSL Library, and how to reference it, in a DSL: http://blogs.microsoft.co.il/blogs/eyal/archive/2009/11/11/dsllibraries-in-vs2010-dsl-sdk.aspx
What else can we have in Domain model libraries?
Domain model libraries are very similar to DSL. As them you can factorize out rules, validation methods, …