You can set the text you want HotDocs to replace a placeholder field with during document assembly if (during the interview) your template user leaves the corresponding question unanswered.
During document assembly, for each unanswered question, HotDocs first checks to see if you set the placeholder field's unanswered text property. If you did not, HotDocs then looks at the component level property. If you left both properties empty, HotDocs then displays the value in the default unanswered text in the Template Properties Dialog.