Basic concepts behind the format in terms of importing data
Importing data via provided import files is incremental.
Only present-in-the-file data is changed and the rest is not modified. Consider following:
translation data is changed only in languages provided in the file (column _language)
if a column is not present corresponding resource property is left unmodified
if a Product attribute column is not present values for this attribute are not changed
if a translation column is present and has an empty value the translation value shall be deleted
templates_elements.csv, section_templates_elements.csv, category_trees_elements.csv are exceptions from the above rules. Since those files represent one resource they are attached to the entire resource(template, section, category tree) is imported at once which means if an element is missing in the file the element will be deleted from the resource if was there prior to import.