Section 10.2 of Vol. 3 (Edition -31), Message Signalled Interrupts, is not particularly clear on the purpose of the Redirection Hint bit in the Message Address Register. The text makes clear that if RH is 1, the destination mode (DM) bit determines whether physical or logical addressing shall be used, and that the broadcast address shall not be used.
However, if RH is 0, it says "then the DM bit is ignored and the message is sent ahead independent of whether the physical or logical destination mode is used." (end of 10.12.1) What is that supposed to mean?
The text also does not indicate the proper value of the most significant 32 bits of the Message Address Register (the Message Upper Address Register)---probably all zeros.
Finally, 10.12.2 says that reserved fields must be preserved on writes. This must be wrong, the fields should be MUST-BE-ZERO. Which agent in the system will have initialized them to a non-default value? The PCI device can't have, since the contents of the Mesage Address and Data Registers are processor-specific. If the BIOS were to initialize these fields, it would enable functionality that the operating system is unaware of. New features are normally enabled by setting some enablement bit, making MUST-BE-ZERO the right choice.