I Identifier Creation Service
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IIdentifierCreationService provides a mechanism to create uniquely named identifiers throughout a range of activities.

Not quite sure what this interface is for. However when I creates a test project I received the error:

The service 'System.Workflow.ComponentModel.Design.IIdentifierCreationService' must be installed for this operation to succeed. Ensure that this service is available.

This blog post http://thorprojects.com/blog/archive/2007/01/17/632.aspx was helpful in getting it to work though.