This attribute may be used to add the prefix characters to the start of:

group names in INCLUDE elements.
Group names in the included file all start with the prefix, to avoid conflict with names in the main file.
field names defined by fields taken from CSV files.
This allows one to read a CSV file without danger of inadvertently redefining fields that were defined in EscapeE or the IDF file. For example, if
then the field names read from the CSV file would all start with XX_ which would distinguish them from other fields.

Mail-merge sample script