The Scripting Index is the definative documentation on all of the elements that can be set, modified, etc. for any of the different functions/display objects.
Here is the JSL from the example for the setting of a check box. The example is taken directlyu from:
Help=>Scripting Index==>CheckBoxBox==>Set
Names Default To Here( 1 );
New Window( "Example",
cb = Check Box( {"One", "Two", "Three"} )
);
cb << Set( 3, 1 );
Jim