It sounds to me like the data needs to be a fixed length.

For example, let's say we need an 8-character string.

If the input is 8 characters, the code outputs the value without any changes.
Input: ABCDEFGH
Output: ABCDEFGH

If the input is too long, it is truncated to 8 characters.
Input: 1234567890
Output: 12345678

If the input is too short, the code pads the value up to 8 characters. ([END] denotes the end of the string, just for the purposes of illustration.)
Input: ABCD[END]
Output: ABCD [END]


http://support.microsoft.com/default.aspx/kb/228738
"The parameter's "COBOL Definition" properties have the Pad with spaces check box selected to specify that the string delivered to the host is to be padded on the right with spaces by COMTI runtime."

http://aspn.activestate.com/ASPN/Mail/Message/activeperl/216...
"This snippet shows how to pad a given string with spaces to a given fixed length."

--------------------------------------------------
Note added at 37 mins (2007-07-30 23:34:21 GMT)
--------------------------------------------------

The HTML on this page reduces the four trailing spaces down to one in my example, but I hope it still makes sense!