The credits feature of Ability FTP Server offers a powerful facility which can be used to restrict how much users can upload and download, to ensure your FTP server is used exactly how you want it to be.
Credits are a feature which is disabled by default but is available to any user or group. They allow you to setup a virtual money system where users can earn credits and spend credits, depending on their actions (uploading or downloading). If a certain action requires credits and the user has none, they will not be allowed to perform that action. A good example of how credits help is in an FTP server which requires users to upload files to be able to download files. Another use could be to sell bandwidth to users via credits, and set a charge for both downloads and uploads. This way a certain user would only allow a fixed amount of usage, and would ensure that your users only get what they pay for.
Another common term used by other FTP servers is the 'upload / download ratios' idea. Although our method has all the same functionality, we decided to adjust the concept as the idea of credits are much easier to understand and use. Also, users logging in to an account with credits enabled can see the current amount of credits they have to spend and are informed of all charges imposed.