Item Type Expression is a text string that defines the syntax to be evaluated by the expression evaluator. The expression input can comprise of numbers, strings, access strings, symbols, and operators. Once you attach an item type with an expression defined, the corresponding value of the expression will display in the element's Properties dialog.
ProStructures properties are now enabled to work with Item Type expressions. This document has listed all the available expressions which can be used as Item Type Expressions to get the ProStructures properties.