(I've split this into its own thread, since it doesn't really have any relation to EBook Columns other than that thread is about a script.)
You can find the list of scripting objects/types here, where ScriptInitData is documented:
https://www.gpsoft.com.au/help/opus12/index.html#!Documents/Scripting/scriptingobjects.htm
The OnInit method which it gets passed to usually uses "initData" as the argument name.
So initData is usually the name of the argument, and ScriptInitData is the type. But you can name the argument anything you want, and it's just a convention to call it "initData" (as well as what the script maker in Opus will give you).