There are two types of lists in Wwise: a drop-down list and a shortcut list (sometimes called a contextual or context list). The drop-down list, referred to simply as a "list" in this help documentation, is a field that contains a series of pre-defined options. To display the list, click the arrow to the right of the field.
The selector button (>>) displays a series of options or actions. The shortcut menu may or may not have a field associated with it to display the option selected. Click the button to display the menu options.