Not sure why, but Zoho has made 'Subject' a system defined required field.  I'm not entirely sure why subject would be required as a key field (i.e. you cannot deactivate it or change it from required).  It doesn't make much sense on many product quotations, (maybe services yes, but products no).  Zoho should change this field so users can opt for it to be required or not required (and therefore be able to remove the field).

Less clutter = better CRM.