This case statement selects the 'correct binding function' that will be responsible for making the formals-actuals bindings. The kind of binding function depends on the tag of the formal arguments of the function: indeed, in Pico the different kinds of formal parameters determines how the actual parameter is treated. Press the links to see the different binding functions. There are two possibilities: either the tag of the formal parameter is a VAR. This means that it concerns a f@x:... function. The other possibility is that the tag of the formals is TAB. This means that it was a 'normal' f(x,...):... function.