6.1.6.4 Versioning and Capability Negotiation
Building TDOs that represent cross-forest trusts requires that both the domain and the forest are running in a domain and forest functional level of DS_BEHAVIOR_WIN2003 or greater.
An uplevel trust, by definition, is one in which both trusting domains are running all Windows 2000 operating system or newer DCs.
A downlevel trust is one in which either of the trusting domains are running Windows NT 4.0 operating system DCs.