What is the difference between Simple, Against invoice and Advance on the Money received screen in Aicountly Billing?

The Money received screen in Aicountly Billing has three tabs for recording a receipt:

• Simple records the payment and, by default, settles the customer's oldest open bills first.
• Against invoice lets you choose which invoices the payment is applied to.
• Advance records money received before an invoice exists. It allocates nothing to bills.

A Refund tab is shown but cannot be used here, because a refund is money going out. Record it on Money paid instead.