Guess you still don't know what people can do with your IC and income statements.
FMB, newspapers are somewhere you can refer to, on the fraud cases happens.
They can use your documents to apply another bank's card and won't use it till after few years.
Principle card already can do this, supp card even safer for them.

Apply on booth or bank's branch?

Is just for showing, it just for the sake of "extra" security to make us feel more secure.

Just attach to the application form   But did you put a cross mark stated for "xx card application only"?