To reference a field in a Subfile the field name by itself is not enough because the name only resolves to a column in the Subfile. To reference a field in a column and row you must also specify a row Index. The index is an integer starting from 1.
For example, to set the value of the selector field in the third row to "X":
SETVALUE("SFL_SELECT","X", 3);
See SETVALUE Function.
To get the department description in the seventh row (note we named this field as SFL_DEPTDESC):
var sDeptDesc = GETVALUE("SFL_DEPTDESC", 7);
See GETVALUE Function.