The append options are only available for memos. 'Append to Top' adds the new memo before the existing memo. In contrast, 'Append to Bottom' adds the new memo at the end of the existing memo. 'Overwrite' replaces the previous memo with the new memo. Note that selecting any of these options in a batch update of a project only applies it to the new invoices created for that project. This means that your previously existing invoices are not affected.