CPA — Cost per action or payment for an action. There are many ways to pay for an
action, and the most popular ones are:

CPL: Cost per lead — payment for an application/action.
CPS: Cost per sale is a payment for a confirmed sale.
CPO: Cost per order — payment for an order.
CPC: Cost per call — payment for a call.
CPI: Cost per install — payment for the installation of an application.

More info here: https://www.hoqu.io/hoqu-whitepaper-en.pdf