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The PrincipalAuthenticationMethod attribute defintion is deprecated because the support for managing multiple authenitcation methods throughout the IdP makes it impractical to expose a single method value.
Dependency Element and sourceAttributeID XML Attribute
V3.4 has corrected some ambiguities surrounding this element with the new <InputAttributeDefinition> and <InputDataConnector> elements, and replaces the old sourceAttributeID setting. Note that sourceAttributeID is ignored once you convert to the newer dependency element syntax, so carefully consider which input data is meant to be consumed by the containing definition.