The subject line in the email that is sent from the form will use the value from the hidden field that contains "form_name" in it. It is okay to have a number before it like "000 form_name" but it needs to be in lowercase.
The volunteer application is already setup to do this, and in source mode it looks like:
<input type="hidden" value="Volunteer Application" name="000 form_name" />
In WYSIWYG mode, the hidden field will appear as a box around the letters "ab". You can right click on this and choose "Hidden Field Properties" to change the Name to "000 form_name" and the the Value to what you want to appear in the subject.