It means that the value you wrote to IPTC:Writer-Editor was truncated
due to a restriction of the IPTC format. If you read the value back again
you will see it is missing some characters at the end.
You may choose to ignore the IPTC specification and write longer
values if you wish, however some (anal-retentive) software may have
problems with values like this. Use the -m option to
ignore the IPTC length limit.