That is not what @Shan is referring to, @eko. @Shan refers to the field label on the sales invoice form.
@Shan, you are using forms for something not intended. (An order is not legally a contract. It is only one of the three required elements: offer, acceptance, and consideration.) Orders have consequences within Manager. Your suggestion would not be the end of the story, but would have further repercussions in tabs and reports. Soon, you might lose track of what you are looking at. In my opinion, contracts should not be forced into accounting records. Generate them separately and attach them if you must.