Windows Server 2003
Object Operation (W3 Active Directory)
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Whereas event 565 logs the permissions requested by user/program, event 566 logs the permissions actually exercised by the user/program after opening it. While an object may accessed several times during the same open, Windows only logs event 566 the first time a given permission is actually exercised. This event is similar to 567 but is limited to Active Directory object accesses.
This event is part of operation based auditing which is new to W3.
You will only see event 566 on domain controllers.