The more canonical reference is in <HIToolbox/Events.h>
:
/System/Library/Frameworks/Carbon.framework/Versions/A/Frameworks/HIToolbox.framework/Versions/A/Headers/Events.h
In newer Versions of MacOS the "Events.h" moved to here:
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/Carbon.framework/Versions/A/Frameworks/HIToolbox.framework/Versions/A/Headers/Events.h