Each coupon is associated with one exam and one customer. It is used right before you start an exam. To obtain an exam coupon, you have to buy an exam. All coupons can be found in the dashboard (you should be logged in to see). It will look similar to following image:
Here, the coupon on the top is associated with an exam that is currently going on. You can just click on the green button and start your exam. The second coupon is associated with exam that has not started and will start in future. When the exam has started this grey button will turn into a green clickable button.
Voucher is used to obtain a discount while making a purchase. The main use for voucher is when a student wants to purchase a package of multiple exams. For example, suppose a student wants to buy a package of 20 exams. We will provide a "discount voucher" that can be used 20 times, which provides 100% discount for 20 exams. He/she only needs to enter the voucher code in the checkout process. To obtain the exam package voucher, you have to buy a discounted package. All vouchers can be found in the dashboard (you should be logged in to see). It will look similar to following image:
You can use this discount coupon while you checkout. To reach the checkout page, go to exam register page, add item to cart and click the cart icon or simply press buy if you want only one item. Here is the image that demonstrates how to use it while you checkout.
You can see that after you enter a valid voucher code and press apply, the total amount will change depending on voucher. If you have bought a package of exam, this total will become zero so that you can choose the exam that you want to sit in.