The compiler determines whether field names are unique within the first 18 characters. This is because some RDBMSs truncate longer field names to 18 characters. (In the case of the OCCURS clause described above, the truncation is to the original name, not the appended index numbers.) If the field names are not unique within 18 characters, the XFD file is generated, but a warning message is issued.
You can, instead of allowing default truncation, use the NAME directive to give a shorter alias to a field with a long name. Note that within the COBOL application you will continue to use the original name. The NAME directive affects only the data dictionary.