Header file \DgnPlatform\LabelHandler.h declares class LabelCellHeaderHandler...
struct LabelCellHeaderHandler : AnnotationCellHeaderHandler {};
Unfortunately, base class AnnotationCellHeaderHandler does not exist in any published header file.
Bump!
AnnotationCellHeaderHandler remains unpublished in CONNECT Update 10 SDK.
AnnotationCellHeaderHandler
Regards, Jon Summers LA Solutions