There's no standard way to represent NULL value in a text file. It would certanly depends on the tool/application they are using to process that file. You'll have to ask your client how the NULL should be presented in the file.

>A client of ours needs some fields NULL.
>We build a text file for them to import into their whatever it is.
>I.E., "myname", 49.95, "productx", ""
>
>They want field 4 as a NULL value.
>can I put .NULL. in there? or store .NULL. to var XVAL
>then build the string like
>"myname", 49.95, "productx" +XVAL