How to make field name optional in FCC

Hi Folks,

I am working on conversion of Flatfile in sender side.

Here my requirement is having the field name in substructure to be optional.

For example:

Recordset

     A

     B

     C

     D

     E

     F

In this I want to set "E" and "F" to be optional.

Because in one set of files these fields will not appear.

Reagards,

Pavithra.R