The parameter function lets you find and use the string containing the parameter of the object or item that stimulated the current response; for example:

response to Districts    # Class of objects

    copy parameter to SelectedArea

If the current response is not stimulated by selecting an object, using the response to or response to item ITEM_CLASS response definitions, ESL returns the null string as the value of the function.

The parameter function is particularly useful in conjunction with classes. This is described in Using Built-in Functions with Classes.