How does EM handle Distribution List migrations to an Exchange 2000 Windows 2000 mixed mode target?
Exchange Migrator 2.x
When Migrating to Exchange 2000 in a Windows 2000 Mixed mode environment, EM creates Global Security Groups. Unlike Exchange 5.5, Exchange 2000 does not include its own directory service; instead, it relies on Active Directory to set permissions on Public Folders, etc. Since you cannot assign permissions to Distribution Groups, it is important for EM to create a Security Group. Global Groups are less flexible when it comes to nesting. Keep in mind that it is a Microsoft best practice to have the AD domain in native mode before deploying Exchange 2000.
When migrating into E2K/W2K domains that are already in native mode, EM creates Universal Security Groups for the DLs.
For more information on groups see Microsoft Technet article:
Q231273 Group Type and Scope Usage in Windows