At the Create Mappings stage of creating a datafeed you are able to specify if you want to Append Data, Append Data Before, or Include in Update:
Append Data: This lets you add text to the end of other text. For example, if you wanted to add text to the notes field but not delete what was in there. It only works with text entry fields.
Append Data Before: This allows you to add text to the beginning of text, such as in the notes field.
Include in Update: This is there in case you need to use the data for mapping but not update the existing data. This is useful where you have incomplete data, like missing postcodes, or have duplicates where you do not want to update certain fields on the existing record, like the current or preferred address.