Custom field allows you to customize the requests that are called upon a successful transaction. For example, if you want to allow your customers to upgrade a specific key, you can call the payment form as shown below:
and add the custom variable to a request by appending [custom], eg.
If the transaction is successful, the desired key will be upgraded (in this case, a specific feature will be added). Note, you can also use [custom] can be appended to the message Custom Message.