Customers pay in three ways. Some buy a licence for software installed on their own machines, and can then choose to pay for maintenance, which brings updates and technical help. Others rent software online as SaaS (software delivered over the internet as a subscription). Once a product is wired into a customer's systems or built into a vendor's application, maintenance and subscriptions tend to renew. Those renewals are the main engine; licence sales add a less predictable layer. An extra copy of software costs little to deliver, so what remains depends mostly on spending on engineers and sales staff. Progress also buys established software companies with borrowed money and repays the debt from their cash. Those deals add accounting charges that shrink reported profit without using cash today.