"Fields," as such, are not a data set attribute. "Fields" in input data are specified by an application. In C, your input record might be defined by this structure -
struct inrec
char field1[8],
char field2[8],
char rsv1[1],
char field3[11],
char rsv2[20],
char field4[4];
I think I got the syntax right; I don't use C very much any more. My field rsv2, for example, is the long sequence of blanks. Of course, you know your data - I hope you do - and your definition might well be different.