A “Negotiable Instrument” is basically a cheque. It’s a peice of paper instructing a bank to pay a certain person a certain amount of money from a certain account at a certain date. It’s negotiable if the person who can get the money is transferrable. Ex. I write you a cheque for $50 and you sign it over to your friend. Your friend can now cash the cheque and my bank will pay your friend $50 from my account.
Here’s a sample bill I found online from an electrical company. The SovCiv is asking if the bottom part is actually a cheque they can cash. It’s just a peice of paper they can mail back to the company that contains all their account information in one spot to make processing payments easier for the company. It might also allow the person to pay their bill through their bank’s bill payment service. Essentially, the bank will use the details on that slip of paper to send money to the company directly from the person’s account.