Strings as Parameters

["Rick Santa-Cruz" <rick_santa_cruz75@msn.com>]

Hi,

I really have a super easy question:

If in a subprogram I wanna use a variable of type String then I always have 
to specify, how long this String will be... for example the following way:
procedure Main is
    S: String(1..10);
begin
...
end Main;

But if I use a String as a Parameter of a function or procedure I don't have 
to specify the range. That means I can just write a procedure like this:

procedure Test_It(S: String) is
begin
...
end Test_It;

What is the technical detail behind this? And why is such possible?

Thanks in advance,
Rick