When mapping from Custom Attributes or Custom Fields, values may contain illegal XML characters or include commas that will interfere with CSV files. In order to use these illegal XML/CSV/TXT feed characters or use product attributes which enclose feed product values with double quotes you can enable CDATA tags.
Adding a “true” after the mapping command will surround the value with CDATA tags (XML files) or double quotes (CSV, TXT).
mapAttribute customAttribute to g:brand true
The above command will map your custom attribute to the Brand attribute of merchant. And, it will add CDATA to the feed for this attribute, allowing to display illegal XML characters while listing products.