In this section:
This format supplies the name of an icon resource (as a standard character string) for the next %bt (Button) button. For example: %bi[tea_icon]%bt[] where tea_icon is defined in an associated resource script (see Icon). In this example no text would appear on the button. However, in most cases you will probably want to supply button text to go with the icon.
For example:
%bi[tea_icon]%bt[Tea!]