caption is a standard character string (can be replaced by @).
In order to change the caption dynamically use the @ symbol and follow a
change in the string variable by a call to window_update@. Alternatively use
the API function SetWindowText together with %hw - Handle.
When used with %sh - Property Sheet in a property sheet, the caption can include an ampersand
(&) to mark an accelerator key.
%mi can be used to set the associated icon.