Thanks to Henk Jansma.
The functions are generally visible in the "Defined Functions" list of the BrainVoyager QX Script Editor (see right part of figure below).
However, not all functions are visible in the "Defined Functions" tab. These are the private functions. The function Rename_DICOM_files_in_multiple_directories() is an example of a private function.
All functions that receive arguments, for example remove_filearray(array) as shown above, are not visible in the function list of the BrainVoyager QX 'Scripts' menu option. Therefore, only the function Launch_wizard_for_functional_data() is visible in the 'Scripts' menu.