From Microsoft:

“WS-Trust will describe the model for establishing both direct and brokered trust relationships (including third parties and intermediaries).

This specification will describe how existing direct trust relationships may be used as the basis for brokering trust through the creation of security token issuance services. These security token issuance services build on WS-Security to transfer the requisite security tokens in a manner that ensures the integrity and confidentiality of those tokens.

This specification then will describe how several existing trust mechanisms may be used in conjunction with this trust model.

Finally, the trust model will explicitly allow for, but will not mandate, delegation and impersonation by principals. Note that delegation is consistent with impersonation, but provides additional levels of traceability.”

See WS-Trust Specification.