My favorite part, getting paid.
An Adjustment is when you need to indicate to the client a difference from the bid to the invoice. For example if model fees are billed direct to the client, but they were included in your estimate you would mark the difference as an adjustment. The Discount button enables you to indicate a discount to the client in the Payments & Credits category. The amount is calculated out of your profit calculation so you don't get a false sense of profit. Extending a discount is a loss against your profit. A Payment, well that's just good old fashioned money in your pocket.
To edit or delete a payment, just click on it and follow the prompts. You can also change the type of payment this way as well.