[quote user="EvanH"]In MicroStaton Connect we would like to use items [/quote]
Presumably, ItemTypes?
[quote user="EvanH"]When you work with tags you can choose whether they should be visible or not[/quote]
ItemTypes data (termed by some item data) are not graphical, and so is invisible until you explicitly construct a text element to display its value.
With tags, a user may inadvertently delete a visible tag element and hence damage the integrity of data in a DGN model. With ItemTypes, it doesn't matter if a user deletes the text element that displays item data.
[quote user="EvanH"]Have I missed something somewhere?[/quote]
Whether or not you have missed something is moot. The published API seems to be missing something. Here's the declaration of LabelCellHeaderHandler...
#include <LabelHandler.h>
struct LabelCellHeaderHandler : AnnotationCellHeaderHandler {}
However, I can find no mention of AnnotationCellHeaderHandler anywhere in the delivered header files. That's going to make it hard to use LabelCellHeaderHandler.