When I write code using the MicroStationAPI I can analyse any linear element using the ICurveVector API. If I want to measure the length of, say, a line-string or a complex string I can obtain it using CurveVector.Length. I don't need to know what type of element I'm measuring.
When I define an Item Type report I need to specify a set of element types that I want to measure. That is, the EC Schemas don't expose, as far as I can see, a LinearElement class analogous to ICurveVector ...
This approach is similar to the model scanner API, where we declare a set of interesting element types (by setting a bit-mask in ScanCriteria) and instructing the scanner to tell us what it's found.
Item Types might be enhanced by providing element type groupings such as LinearElement, AreaElement and VolumetricElement. For example, the LinearElement grouping would include line, line-strings, complex strings, B-splines and arcs.