dml expression means abinitio dml are stored or saved in a file and dml describs the data interms of expressions that performs simple computations such as files, dml also contains transform functions that control data transforms,and also describs data interms of keys that specify grouping or non grouping ,that means dml expression are non embedded record format files

.xfr means simply say it is non embedded transform files ,Transform function is express business rules ,local variables, statements and as well as conn between this elements and the input and the ouput fields.

Ramesh Charugulla. 

  The main difference b/w dml & xfr is that

      DML represent format of the metadata.

      XFR represent the tranform functions.which will contain business rules

DML describes the record format of the data. It is metadata.

XFR is the set of business rules called a transform function.