Help: Unfamiliar type declaration

        In a program I am modifying, I have run across a type declaration that
        I haven't found any documentation on.  The code is as follows:

             WinType : Ag..Nul;

             array [1..Constant] of WinType;

        In the code I have seen an A and a D (not with '', if I am not
        mistaken).  The problem is this.  I have tried to assign an X to
        an element in the array but I get a compiler error saying that it
        is not valid.  

        If anyone knows what the legal values are for this array and where I
        can find information about this type, it would be greatly appreciated.