CAD tip # 4805:

In the dynamic block editor (BEDIT) use the "Define attribute" icon (or the ATTDEF command) and in the attribute value definition click on the icon "Insert field". In the field category select Object > BlockPlaceholder and in the properties list select the requested (watched) geometrical property of the block. You can add more parameters to the properties list (including lookup parameters) with the Parameter function (BPARAMETER) in the block editor. Such user defined named parameter will be added to the block properties list. You can also define the display format for the computed attribute.