As you may have experienced, articles may be rejected if the short summary is not original. A short summary should not be cut-and-pasted from the article you have written - especially the first paragraph. You should strive to summarize your whole article in a new way in the short summary. The short summary is there to give customers a brief overview of the information contained within.
This is their first look at your article. Make it count!
If you simply paste your first paragraph into the short summary box, when they get to the long summary, they'll have to read this information after they've already read it once. This is a waste of time for customers, who may pore over dozens of articles searching for those that fit the needs of their sites precisely. Not only that, but a first paragraph is often much lengthier than a short summary should be and doesn't always convey the central idea of the article. One to three sentences is probably the ideal length for a short summary - this will allow the customer to quickly get an idea about the article's pertinence to his or her needs.
A short summary is also where you would include any extra information that would be useful to the customer. For example, your article may include references or a table to illustrate your point, or be written to the customer's specifications. Don't, however, include any promotional language, as this is one practice that Constant-Content will not allow.
If you've got a special note to the editor - like "Article was previously submitted for a Private Request. The customer no longer needs this article, so I am submitting it for general purchase." - you can put it in the short summary as well. Prefacing it with "Note to Editor:" will help grab our attention.