Item type is really a powerful tool to add metadata to an element. the main problem i see with item type is its not user friendly. user need to have good programming background. And Yes there are available syntax in the dialogbox but its not working to what we need some time especially when drilling to extruded element or elements that has geometric complexity.
what i suggest is for bentley to develop that when we right-click a specific element property an option there to copy the expression, so user doesnt need to guess the correct one.
another option would be is similar to TEXT DATAFIELD, where it allows user to map a text through an element property by selecting the property in the element information.
We have lots of complex elements where we need to have multiple related instances (parent to child) in multi level that is really difficult to create an expression for a normal user. hence we are suggesting this to be more easy.
another option is it would be nice if you can just select from the element properties > rightclickt it and an option to automatically creates an expression for you no matter how complex the element is.
example attached where, the complex shape is built with 2 line strings and 2 lines, and we want to create an expression to grab one of the line strings total length.
this is modeled differently from a normal Path + shape extrusion.
An Item Type definition is meta data; an Item Type instance is real data attached to a DGN element.
You don't need to be a programmer to use Item Types. When using an Expression to derive an Item Type then, yes, it's useful to be a coder. However, the Expressions dialog does not provide a guide to writing an Expression, and doesn't provide good diagnostics.