Domain Functional Levels query

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My text says that the default Windows 2000 mixed functional level supports features available in Windows NT 4.0 while Windows 2000 native supports Windows 2000 DCs but not NT 4.0. It seems there's no functional level at for domains that supports NT, 2000 and 2003. At the forest level, the default Windows 2000 forest functional level supports all three. Can somebody give me some clarification here please? Thanks.