In a structured expression, you can call an operation defined in the model. An operation from the model can be either built-in or custom. The built-in operations, such as Union, Find, or Filter, are stored as executable opaque behaviors in the Built-In Operations package, which is owned by UML Standard Profile::MagicDraw Profile.

Built-in operation types. Selecting to create new Exclude operation

Be sure the Exclude Used Projects command on the Filter Options button menu is unchecked!