A reference code is a kind of ID used to refer to a given transaction. Before someone buys your digital goods or products, a unique code is generated. This code is then used to "back track" through the process of buying a file.
From a users perspective, they can be helpful if something goes wrong with their payment gateway or the url is mangled when being sent through email (word wrap).
From a merchants perspective, they can be used to "look up" a given transaction and alter its status (for example, increase the download count.
The download count is the number of times your customer is allowed to download the file before the reference ID becomes invalid. This is used to protect against link sharing. You can adjust it in your control panel under:
Configuration -> Main -> Max Downloads
We recommend 5 attempts, just to be sure people are able to download the file if something goes wrong, but you can probably set it to as low as 1 or 2 if you're prepared to field buyer complaints.
To protect against link sharing, download links become invalid after a period of time, the default is 7 days (one week).
You can change this in your control panel by selecting:
Configuration -> Main -> Max Days
If you set it too low (for example, 1 day) your customer may have problems downloading the file (for example, they leave the office for the night) we recommend a period of 7 days, 1 week is ample time for most people. Setting it too high might result in people sharing the link with others.